Like some, I have a mixture of seller's and buyer's remorse.
Sometimes sad I let go my childhood hot wheels from the 1960's and early 1970's - lots of mint and hard to find cars, plus they were mine. The $2k I sold them for then was good, but I'd rather still have them.
Buying, sometimes it's hard to pass on a coin that I know is a great deal, though I already have one in my collection. I know I can resell them, but for the time and effort needed, some self control would be better.
I think everyone who's dealt on
ebay regularly has been sniped at the end of an auction. Sometimes I'm relieved to have saved hundreds of dollars, and other times it's a let down when I really wanted the coin. But, I always remind myself I didn't lose by $0.01 or $1 or $5, given the buyer probably would have bid a lot higher than the final price.