I admit to being hopelessly caught by Amazon.com, and not just because of the the name, lol. Their used offerings are really good, too, and I've never had a problem with one of the used sellers ... in fact, they're usually faster than Amazon!
You might try http://www.paperbackbookswap.com. It is more of a trading site than a purchasing site. The way it works is, you list the books or audio books you are willing to swap, and everyone else does the same. When someone requests a book, you mail it to them, and you get one credit to your account. Then you can use that credit to request a book from anyone else on the site, and they mail it to you. You can print bookrate postage right from the site, too, so you don't even have to go to the post office to mail it.
One of my best friends uses it, and always has shelves of books waiting to be read. She loans me piles at a time.
I really ought to sign up and part with some of my collection of books. I have such a hard time getting rid of books, even ones that I didn't love! I think a site like that would make it easier, knowing it was going to someone who really wants it. I've taken boxes to the library to donate before, telling them to keep anything they want for their collection, and put the rest out for free for anyone who wants them. I've taken books to Goodwill occasionally, too, and to 2nd hand book stores, but it is a real time commitment to do that. It wouldn't take much to keep a shelf of listed books, and stick one in a padded envelope and mail it when someone requested it. :)
I don't buy many books any more, I mostly get them from the library. But any that we do buy we later give away or sell to Half Price Books. I get a few dollars to spend on other books there. And if I have some books that they're not interested in, they'll accept them as donations.
Half Price books started in Texas, but I remember going to one in Wisconsin a LONG time ago. Mostly old paperbacks, but I got some great deals on old sci-fi novels. Original price (back in the 50s) was $1.20 for some of them, lol. Half of that was nice!
It was a great little store but went out of business.
There is a HP book in Greendale, WI. It's nice but the books are too new for me, I like those old musty books. lol