Last I regret was a few years ago, someone contacted me about a Fathom they had for sale. Told them I'd buy it for $100, and they put it online a few hours later for $150 and a friend saw and bought it immediately. Still happy it went to someone I know..
Most of the people that contact me just want to get a price quote to sell it somewhere else, so usually I don't offer much as it's almost games I'm not really looking for or have space / time for at that time.