My husband and I have tried to sell our used books at a couple of different book sellers--most are particular about what they'll take and sell. I'm ok with this.
but i remember reading somewhere about an online company that buys used books and pays for the shipping? i remember it wasn't much per book (a $1 or less usually depending upon the title), but i can't remember the links.
I don't know of one that buys used books, but I know there are a few that are for trading used books. I've had a wonderful experience with Paperback Swap. It's not just for paperback books, any book, even audio books qualify. It doesn't cost anything; you just have to list at least 9 books to get started and then people request your books and you pay the shipping to send it out (around $1.59 on average per book). Then if you want someone else's book, they send it to you. The site is very user friendly and everything is done for you; everything is tracked and you get reminders, etc. and it generates the mailing label so you just have to print it, slap it on, and pay the postage (it will tell you how much the postage is, so you don't have to stand in line to get it weighed, etc.)
ETA: If anyone decides to join after reading this, please use my referral link. That way, both you and I will get one free book credit!