Are you familiar with bookfinder.com? I would start there. Type in the book and you can see how many copies are currently for sale and the prices they are listed at. That will give you an idea of what you have. Probably best to figure on the bottom prices listed.
If you have anything truly desirable, EBAY will probably get you the best price. Listing a few books will also give you some idea of the market, it's amazing how few buyers for some things there are out there.
If you just want a dealer, try Abebooks, search for one of your books, you'll get a list of copies for sale. You can click on the seller and you will get their contact information, that will get you a good list of dealers.
Good luck, I don't think this is a very good time to sell books.
Tim
