That last point is debatable. The main argument against such contracts is that people don't have the physical capacity to hand over control of their own bodies to anyone else, and you can only enter into contracts which you can actually fulfill. Regardless, the "usury" contracts you're attempting to undermine have all the necessary properties of a valid contract: consideration, offer and acceptance, capable parties, mutual assent / "meeting of the minds". Their only failing, apparently, is that they don't meet with your approval. Enforcement of valid contracts (whether or not they are recognized as such by local law) is very much a condition for a free market. Arbitrary interference with voluntary contracts would undermine basic property rights and render the market non-free.
Contracts of any sort are in no way a "basic property right", nor necessary for a free market. The power to contract is not only a different right but an entirely different class of right than the claim rights that constitute ownership of property.
A claim right is a right to have someone else do or not do something -- such as the right against people doing things to your property against your will. A liberty right is a right to do or not do something, which is to say, the absence of claim rights against you. The second-order versions of those are respectively immunities, claims against people making changes to your first-order rights; and powers, liberties to change first-order rights.
To have a free market, all you strictly need are claims against anyone acting upon your property against your will (which is to say, private property), all liberties compatible with such claims (which is to say, general freedom), and at least the power to transfer ownership or your property to someone else (which is to say, the power to trade), consequently changing who has what claims and liberties to what.
Any other powers beyond that, or immunities against such powers, are open for debate without putting the freedom of the market in question. The power to contract is one such power beyond that, and what its limits should be is open to debate without putting the freedom of the market in question. I actually lean heavily toward the position that there should be no powers beyond that power to transfer ownership, maximizing immunity, just like there should be no claims beyond the claim against anyone acting upon your property against your will, maximizing liberty. I'm not arguing for that extreme here now, but I do have a deontological argument for it; however, I can also see consequential arguments against it that I'm still mulling over.
But there are very clear consequential arguments to at least limit the power to contract in a way that undermines rent and interest, though -- the ones I gave earlier in my first post you responded to -- and while we can argue about the strength of those arguments, it in no way would undermine the freedom of any market to have no power to contract at all, much less just no power to contract usury. It would undermine certain widespread institutes of capitalism, but that's not the same thing as undermining a free market, and that's the whole purpose here, to get rid or capitalism while keeping the free market. We can argue about whether the consequences of that would be good though, if you like... which it looks like you've already begun to do below.
I, for one, very much want to be able to enter into contracts which involve me paying rent or interest with the expectation that the terms will be enforceable in the event that I should default. The alternative is that I won't have access to loans.
I wonder, what do you think people with all that money will do with it if they suddenly can't get any use from it by lending it out at interest? Now they have useless piles of money doing them no good; certainly they'd like to find some other good use to put it do, what do you think that might be? Honest question; I've done the analysis of this mostly concerning real property, which would otherwise have been rented out, as well as with chattels that might have been rented, and I've found that there are alternative arrangements involving only sales that satisfy the economic needs that rent is currently serving without the harmful side-effects I'm looking to eliminate. But I haven't done a detailed analysis on alternatives to the rent of money (i.e. lending at interest) yet. So I'm curious to hear you try it. If you had enormous piles of money, more than you could ever spend on yourself in a lifetime, and you couldn't collect interest on it, nor collect rent on anything you might buy with it, what would you do to get any good out of it? Bearing in mind especially that there would simultaneously be suddenly a lot of people in need of money, that you might be able to come to some mutually beneficial arrangement with. This is not a leading question, I want to hear your realistic thoughts on the matter.
If I object to the practice, I always have the option of not entering into such contracts; I would not thank anyone for taking that option away.
But you, and everyone else as well, have to live with a market distorted by the presence of such contracts, in much the way that the legality of slavery contracts would distort the market for free labor. It's precisely those market distortions I'm looking to get rid of.
My specific motivation in developing this in the first place was observing that real estate has value as a source of rental income, which drives up the sale price of real estate (because owning more than you need to for your own living space is a source of income, not just a store of wealth), which in turn makes it more difficult for people who don't already own property to get any at all, leaving them stuck renting even if they'd rather own, to the ongoing benefit of those who already own enough excess property that they can rent it out, and at the ongoing expense of those who don't even own enough to live in themselves.
That's exactly the kind of having-capital-makes-you-money, lacking-capital-costs-you-money wealth-concentration cycle that I'm trying to cut short. If rental wasn't legally possible, then the would-be-rentier property owners would be forced to sell their property at prices and on terms that the would-be-renters could afford, i.e. comparable to their rent payments. (There are a lot of other nuances necessary to make that economically viable, that we can go into if you really want, but that's the bottom line). The possibility of rent distorts the sales market that would exist in the absence of rent, to the detriment of those who don't own property, and the benefit of those who do, in a way that simply "not choosing to rent" doesn't let you escape. I choose not to rent, but am in practice unable to act on that choice because of the distorted market that the possibility of rent creates.
Oh, that's good. In that case you should have no problem with contracts involving rent, since they are in fact exchanges of "actual, real things"—the service of allowing access to the owner's property in exchange for the renter's money.
Allowing someone to do something is hardly "a service".
An analogy I like to use is the idea of selling you a right to punch me in the face. Granted that I have a claim against you punching in me face against my consent, I can change whether or not you are allowed to punch me in the face by changing whether or not I consent to it. You also have the power to transfer ownership of some of your money to me. You transferring such money to me might motivate me to consent to it. But your transferring such money to me cannot give you the right to punch me in the face, otherwise my ownership of my own face would be compromised. I retain at all times the choice of whether to consent to it or not, and thus control whether or not you are allowed to punch me in the face, so you cannot have a right to do so. I cannot give you the-right-to-punch-me-in-the-face-whether-I-consent-to-it-or-not without giving up ownership of my face, selling it to you.
I can't actually sell you my face because I'm, as you pointed out, unable to actually surrender control of it to you. (Well, I guess I could cut it off and sell that to you, but you get what I mean; I can't sell an integral part of myself). But the above is analogous to renting out any other thing -- a physical thing, not something abstract like "a service" or "an idea" or "a right", but a real, tangible, material object. If I own it, I can change whether you can act upon it by changing whether I consent to that or not. You can transfer ownership of your money to me, which might motivate me to consent or not, but I can't sell you the right to do something to it whether I consent or not, without selling it to you. To have rights to a thing is to own it. I can't sell you rights to something and still own the thing myself. Well, we could perhaps jointly own the thing, and thus both have claims and liberties against each other as to what we can do to it; but then if I wanted exclusive ownership back, I'd have to buy you out, at the price of your choosing, and rental is nothing like that. Rental purports to, in effect, sell a thing and yet still retain ownership of it. It's a nonsense arrangement.
To eliminate that possibility you could enter into two separate contracts, one to transfer all the rights to the other party now, and another to transfer the rights back at the end of the rental period. In the interim ownership would lie entirely with the renter. You'll still get your property back in the end, but you can't terminate the rental early.
That's actually very similar to my proposed alternative arrangement to rentals: you sell it (usually at a high price and on long terms) and then you buy it back (usually at a low price and on short terms). For short-term use of things, this very much resembles rent as it exists already, and the profit made by the "rentier" is tied directly to the convenience of such a business to the "renter", and quite justified. Because, since the "renter" is actually buying, he owns the thing after the initial sale, and if he likes could sell it to someone else for a higher price than the "rentier" is offering to buy it back, or just keep it forever; he's under no obligation to sell it back to the "rentier", only to finish paying for it, whether by selling it back or selling it off or some other means that allows him to keep it himself. So for longer-term use of things, this "rental" arrangement looks more and more like a sale; or rather, since they're all actually sales technically, shorter-term pairs of sales look more and more like rentals, but they're technically not.
In such an arrangement, someone "renting" an apartment is actually slowly buying it; they can cut out at any time by selling back to the apartment manager, likely at a net loss for the whole deal, which is how the apartment manager makes a profit, but which may be worth the convenience to the "renter", just like paying actual rent money in exchange for no property is a convenience worth it to some actual renters. If they're in no rush though, and kept the place in good condition (maybe by subscribing to the maintenance plan the apartment manager offered with the sale? or just doing their own repairs if that's a better deal?), they could find another person to sell it to, on similar terms to the ones they're buying for, and end up paying close to nothing once it all winds down; which makes sense, because they ended up with no property to their name for it, so why would they have paid anything for nothing? After a few decades of long-term "rental", the "renter" will have paid enough in "rent" that he now owns something somewhere; unless he really needed the convenience of moving quickly a lot, in which case he may have sacrificed some of that potential investment for that convenience, but that's his choice. Those who want to "rent" in a way like we rent now can have effectively that; and those who want nothing more than to get out of renting have a gradual path to ownership to reward their patience. Who loses? Besides the wealthy people profiting off of those who are perpetually stuck renting when they'd rather own, that is, but they're the problem I'm trying to eliminate here.