We have dome fairly well at Amazon - huge numbers of vintage engineering texts in our library, as well as specialized craft books (weaving, etc). We have sold books up to about $125. If you have time, it is well worth your while to type in a few hundred isbn numbers and see if there is any "good stuff". You are looking for books with a resale value over 99 cents (or so) and a "popularity rating" of less than 1,000,000. (The lower the number, the better)
DH bought a book at the airport (full price) read it, and said it had to go. We got about $6 back. This is about number 6,000 on the Amazon bestsellers list.
"1421: The Year China Discovered America [Paperback] by Menzies, Gavin"
If you combine Amazon or half.com with
www.paperbackswap.com (the junk goes to the latter site) you should be able to always get the reading material you want, at a fairly low price. If you really LIKE selling on these sites, then start hitting up your family and friends for unwanted books.
This has had 2 non-monetary benefits for us: 1) we have space on the book shelves for our books now, and 2) in sending away unwanted books, we find some treasures well worth keeping.
-sleeplessMommy