He deals in non-obscure books with known pricing and demand. Your hard to find, obscure book won't be on his shelf. And on the off-chance it is, he's pricing based on other vendor prices, so it won't be cheaper.

So... consider him a saint all you'd like, but he is not helping you in the way you imagine he is.

All he's doing is going to thrift shops, and ensuring that poor people don't get any of the popular books at below market prices. Nothing saintly about that, IMO.