I don't sell games too often, but just about any game would go at the right price. I figure there's a formula to it. If the offer is high enough that I feel I could replace the game with ease and pocket enough extra money for it to be worth my while, it could happen.
However, the two games that I get the most unsolicited offers for are Mystery Castle and Ghostbusters. I get inquiries on these all the time, but they would be the hardest to let go. MC would be very difficult to replace and GB would be impossible, so it would have to be sick crazy money on those at this point.
Brian