Back in '07 I sold my original '84 Prime AFAed 90 for $8,500-.. Looking back, I should have kept it! It had the sentimental value that money can't possibly replace.
My first trade here was for some starscream figs and parts for my G1 TM Scourge. I had no particular love for him but had I known at the time of his rarity I would have not done it. Live and learn.
More regret losing Optimus prime and metroplex ended up a friends grage 20 years ago and he still may have them
I sold a MISB G1 Ironhide a few months before I met my girlfriend who turned out to be an Ironhide fan. I eventually got her an Encore Ironhide, but I still kinda wish she could've had the vintage one I'd had sitting in storage for the last twenty-odd years. I also regretting selling my sealed G1 and G2 Bumblebees. But then I recently found them in storage and remembered I'd considered selling them with a load of other stuff ages back but had obviously held onto them in the end. Sometimes a poor memory can bring you later joy.
I had 2 supreme class starscreams and i sold one, i regret selling it now because that one item i want to keep MIB, and i really want to check him out. Another regret is i had 2 primus's as well one with the unicron head and i bought another one to open because i will not open the one witht he unicron head, and guess what my dumbass sold the regular primus, so still to this day i have never seen a primus loose or transformed him, I hate that about myself. sometimes i will get 2 of a figure so i can open one and not open the other (i don't do it very often) but everytime i do that i end up selling the effing one i'm plan to open.
One year at Botcon, I got a boxed Diaclone blue blaster and due to lack of space in my luggage I sold it to a friend (YOU KNOW WHO YOU ARE). But as it turns out I found another one later so it all works out in the end. Now we both got a great vintage toy.
I regret letting my parents sell most of my TFs from childhood at our garage sale earlier this year, but they apparently went to our neighbors who have three young boys, so I guess its OK in the end. I can't be selfish with my childhood memories...