Note: Messages that say "I would like to buy this game at your asking price, how can we make this happen?" are more likely than messages like "Do you still have it?" or "I'm interested, how long have you had the machine?" to be responded to quickly.
In any case, people do have lives, sometimes I won't get a message back for a few days. Beyond that I figure it's sold, the seller doesn't like me, or the seller has decided to keep it and I forget about it. Plenty of games out there.