As to your used battery idea, it is not a good one. Most used batteries are car batteries.
Aside from the fact that we're talking about used EV batteries, it might interest you to know that high-end vehicles are now replacing their flooded lead-acid starter batteries with Li-Ion packs. Even a really dinky one is capable of starting the vehicle, but the truth is that there's embarrassingly more electrical accessories in the modern car, and they need a battery with more capacity so that you can use them all at once even in a vehicle with a stop-start system. As the price on electrically-operated accessories (like power steering, heat pumps, and so on) comes down due to economies of scale, you can expect this trend to proliferate down to cheaper cars. It's pretty hilarious to look even into older vehicles, say a saturn, and compare the size of the battery to the size of the engine. Which block is bigger, the battery box or the cylinder block? A Li-Ion battery would be half the size, and let you design a foot off the length of the vehicle — or put it someplace else, where it would do you more good.
Anyway, these Li-Ion packs can be broken down and their individual cells tested, matched, and re-used, so the car starter battery of the future will also be useful for these systems... just not the batteries of today. Those are already aggressively recycled, however, like most car parts.