One of my favorite online marketplaces has to be Half.com
For those of you who don't know, Half.com is a division of eBay, but it's not run auction style, rather people just list their items for whatever price they want and wait for someone to buy it at that price. The other difference is that they only deal in media, i.e. books, movies, music, etc. The prices are very competitive, and I've gotten many good deals on textbooks here. Also, for all of you college students looking to get more than $5 for the textbook which you paid $100, this is a great place to unload your books
. There was even one book which I somehow sold for more money than I paid for
! The only downside is that things might not sell immediately, but if you keep your prices competitive, they'll sell eventually.