My preferred method of selling/getting rid of books, in order of preference:
1. eBay for high dollar books that are more or less guaranteed to end well, or sets that seem to be in demand. Auctions are dead (unless you have something truly valuable) so BIN with BO option is the way to go these days. But as has been noted many times, be prepared to be hit with eBay, ebay shipping, and Paypal fees. You're looking at losing at least 13% of what you "make".
2. Here for stuff that I think users will be interested in. Boardies are pretty stingy though, so although I have to pay fees through eBay, I will usually start there, then come here.
3. LCS for what is left after eBay and the boards.
4. HPB for what is left from LCS, though you should be warned, they don't pay much. For most moderns you are looking at about 10 cents a book, maybe a quarter. They do pay more for older books, and sometimes you can work that to your advantage. They aren't exactly pro graders, and tend to be more lenient in grading when looking up what a book is "worth". I've been able to dump pretty common bronze books and beat up silvers that no one else wanted for $5-$10 a book, just because they tend to thin "old = valuable." But again, beware, for the most part you are practically giving them away when you take most books to HPB. Tax write off may be a better option for you.