Where are you selling it? If on ebay it's what ever they are selling for same as in Craiglist or Amazon. Problem with Macbook pros especially top of line models is they don't retain their value as much as the small ones. Why pay $1500 for a 2 year old computer if you can afford a new one for $1800-2000.
You can make a bit more money off them if you leave programs/music/movies on your HD.