Amazon.com Marketplace – Easily the highest traffic of consumer book buyers. Easy to list. Free listings, only pay when your item sells. You get paid every two weeks. Their tools are pretty limited unless you pay $39.99 a month for a Pro Merchant subscription, which gives you a wide range of extra tools to sell your items. You’ll need to be selling 40 books a month to make this worthwhile, however, so you should start with the free selling option and upgrade when you’ve expanded your inventory enough to make Pro merchant work for you.
Ebay.com – Best place to sell rare and collectible books to the average consumer, and also a great way to sell a “Lot” of books — that is, a group of books. This is something you cannot do on Amazon Marketplace. Since shipping 5-6 books is fairly inexpensive, you can give the buyer a great perceived value and you can move bunches of books quickly. One trick that seems to work well is to make one of the books a high-value book and surround it with lesser-value books.
Half.com – This website is part of the eBay Empire. You list books by ISBN and it’s pretty simple to use and get paid. They send you the order of where to ship the book and give you your cut.