I regretted selling one of these letters http://rayallencollection.com/Genuine_1_1.php Got it back again for what I sold it for when I saw it was for sale though, so that is one good thing
I have a few, when I started collecting in 04 the first time, I wouldn't pay anything more than $1 for a card, passed on 2 Tim Duncan 97-98 pmgs for $50 each. Then 2 years ago passed on a green 97-98 pmg for $2000. If only we had Marty mcflys almanac!!!!
Most things I've sold in order to acquire something else. Some I regret just because of how beautiful they were, but in the end I wouldn't have had what I had without doing so.
I do regret buying some things though, especially when I need money now and can't move them!!