Yes, so many places! First off, if you want other manga/comics/other books, you can trade them one-for-one at
www.paperbackswap.com (if they ask who referred you... ringwraith10!
)
Selling sites:
Amazon.com, of course
http://www.abebooks.com/http://www.alibris.com/Also, I think the above sites will all buy your books themselves if you don't want to go through the hassle of selling them to a buyer yourself. Of course, they won't necessarily pay as much.
But seriously, I preach the heck out of paperbackswap.com all the time. I've gotten some really good books (anthologies, art books, D&D Manuals, etc.) on there and every book is worth just one credit -- so if you trade away a $5 paperback you'll receive a credit that you could then use to get a $50 art book. And you don't need to feel bad -- the person getting rid of that art book knows what they're doing and for whatever reason just doesn't care.
I mean, I've traded away textbooks that I paid over $100 on because I didn't want to deal with them and the bookstore wouldn't buy them back. So everyone wins!