If you have books that are unusual and sought after, you have a good chance of making something. If there's an ISBN, as others have said, you can check Amazon and half.com. If you want to see a broader view of an unusual book, you can use bookfinder.com, which searches many, many booksellers. This is also a useful place to do research if you don't have an ISBN. Just beware, some of those books can be *very* overpriced - review these with a skeptical eye!
Remember that half.com gives you a shipping allowance -- though if you've got hard cover books, that allowance hasn't always covered all my costs (and I use mostly recycled materials!)
After you have a general idea of the value of the books individually, you might consider bundling - especially if you think a certain group would make a good holiday gift to sell on ebay or craigslist.