Too many variables to give a concrete answer.
How hold is the MBP, what condition is it. what configuration do you have, does it have applecare. What's the local market and/or ebay market.
The best time to sell a MBP is right before apple releases the new ones, the problem is taht apple doesn't tell us when that will be.
Right after the release ebay/craigslist gets flooded with the older machines as people upgrade. Lower demand and high supply dictates a decrease on what you could possibly get - how much is just pure speculation imo.