I've only sold/traded one gun but I have future plans to replace it anyways when I sold it. It was almost right but not what I wanted exactly, so that was okay.
But I have missed buying a gun and regret it everytime I see the price on the same type again. Missed a Kahr Thompson 1927 semi auto for real cheap, missed a FAL for real cheap, so now, I tend to buy on site... that might be worse though 'cause then I see a slightly better deal the next week! (Though if I didn't buy the first deal, I probably wouldn't have seen the second deal!)