Half.com and don't forget Amazon.com
Search for your book on Amazon.com
Make sure you find the listing for the exact issue, release date.
Look to the right, under the "More Buying Choices"
Review the condition and asking price of those already being offered, and see if you can justify the best price for your condition.
One caution, Amazon.com sets the postage, so be sure you look carefully and if need be, adjust your asking price to cover any really heavy books.
I sell quite a few books and CD's on
Amazon. Why? I like the 60 day listing, I like the fixed price and how I can clearly see all others being offered. I like being able to adjust my asking price at each re-listing (no fees if not sold). And finally, I feel I get a higher class of buyer, as opposed to ebay and half.com.