Two stick out for me.
When i was around 13 or 14 we had a garage sale. Wanting some omoney and not knowing the attachment I'd have latter I sold some of my more beat up Megos, a remco Wolfman, and a wooden box filled with just about every kind of trading card you could imagine. I MIGHT have made $20 that day. Luckily I did already have an attachment to my Mego Spider-man (since I had since I was 6) and he avoided the sell-off and is still with me.

The second, was in my mid to late twenties I sold a handful of my original Empire Strikes Back/Return of the Jedi figures to a store as well as my large size R2-D2.
That one really stung year slater as my ESB figs in particular went on many vacations/sleepovers. Many years later I went back to the same store in search of my figures knowing there some identifying marks to indicate if they were mine. I found two of my favorites and was thrilled to get them back , but the rest are lost forever. I can't even go back to that store for another try as they have since closed.