I'm going with the new MBP and selling my MBA which I got on Feb 10th or 11th. It is in brand new condition still... I dunno where to try to sell it, or how much to try and get for it. I'd like to avoid EBay.
Either craigslist or the marketplace here would seem to be your best bets if you want to avoid eBay fees. As for price, Apple sells refurb units for 85% of the cost of new units, so that should give you some indication.