More often than not the books for sale at the thrift store were donated, which means when they sell the store makes 100% profit (minus overhead). In my experience, however, some thrift store owners like to look at themselves as better than others because they are operating a "charitable" organization, more often than not with a religious organization backing (and providing tax shelter) the thrift store. These institutions claim to be helping the poor and the needy, when in fact they charge the "poor and needy" customers the same price as anyone else who shops there.
If they determine the customer is shopping with the intention to resell, they typically react negatively. I have been banned from shopping from a local thrift store for no other reason than the owner had learned that I had resold items on eBay. From the owner's point of view I had taken away opportunities for less fortunate people to purchase these same items. Here are some additional details, however.
I had already learned from employees at this thrift store that they frequently received more items than the building could contain. Each week, they gathered up items in the store that had not sold (I believe the items had about a four week period before they were gathered up). If they were glass, they were smashed. If they were clothing items they were bagged and prepped to be shipped off to a company that shreds unwanted fabric and packs it into insulation used in the manufacture of automobiles. I presume the glass was sent out for recycling. From what I gather, the company that made insulation paid the thrift store for the fabric and covered all shipping costs.
My point is - if we were denying the poor the benefit of obtaining these items, they were being replaced each week in such volume that would result in a significant amount of the items being destroyed/recycled/sold to a third party. So the reality was that there was more than enough to go around.
Another point of view is that we were taking advantage of the thrift store by reselling their product for a higher price than what we paid. I fail to see how this is a problem. What anyone who does this is doing is work. It takes time to sort through items in any resale environment and determine which are valuable and which are not. Any thrift store owner or employee knows this. It also takes time to take those items into a different forum. For example - to list an item on eBay it is typically necessary to provide detailed photographs of the item in question, create a listing and respond to questions about the item. Upon the completion of the sale it takes time to properly package and ship the item. So in effect, it is not that the item itself is being sold for a higher price, it is that the resellers are being compensated for their time, which is, in effect, a service.
My final point is that when the owner of a thrift store, yard/garage sale, or library gets offended that someone is reselling their items, it is hypocrisy. These individuals who are offended are already engaging in resale. Of the three, the thrift store owner is the most guilty because in most cases he or she is reselling product that was given to them freely as a donation. Unless the thrift store is being operated as not-for-profit and all proceeds are being donated to charity, they are usually making excellent money from a small business owner's perspective. In our current economy, thrift stores are one of the few business models that are doing rather well. Therefore when these individuals become upset with or feel threatened by resellers who purchase their product, it is ultimately a problem of greed - they do not like the idea that someone else will sell an item for more than they (the thrift store owner) was able to sell it. I have a simple answer for these people - try reselling these items on eBay or Amazon. Hire the staff to do it if you do not have the time to do it yourself. I predict, however, that the profit margin will not be as large when compared to the overhead of hiring people to do this and the amount of time necessary to invest in order for it to be successful.
If the above offends, perhaps capitalism is not your bag, baby.