I have done quite a bit of "research" into this within the past few months as I had/have quite a few textbooks, hardcover, and paperbacks that I wanted to get out of my house.
The textbooks sold on
Amazon easily and quickly (I was impressed). I went with Amazon because I found it easier than Half.com from a sell/mail point of view. Amazon takes a "large" cut but after half.com fees and paypal fees, it was about the same.
The hardcover and paperbacks I found to not be worth much- I would get around $2 (after fees/postage) for each book sold. In some cases, I would get less than $2 per book. It wasn't worth my time or gas to go that route through Half.com or Amazon.
I posted the majority of the books on paperbackswap and have since mailed out 3 books and I am waiting to receive my second book in the mail. I see it as paying $2.77 for a new book (the cost to mail my old one).
The only other way to make some cash is to have a good number of books by the same author and post it as a lot on Ebay. I have all 11 books in a series that I'm going to list this week the average for the set is $30. After fees, I'll still be making slightly more than $2 a book.
So, the moral of my long post is, do some real research into what each book is selling for at the lowest price, how much it will cost to ship, and what your true earnings will be.