As far as retro stuff goes
Sometimes something is in far worse condition than one might think, breaking even or a small profit is bad deal for me. It's not super uncommon to have some part in a worse condition than one might think or a broken console wich isn;t a big deal really since usually it's just a small part of a set but for an entire lot to end up like trash that's a true rarity I lost a couple tenners I was greedy at the time since I would usually not buy such a lot but I saw potential and it was dirt cheap or so I would think, the trash can was my best friend though not worth the effort
I sometimes even regret buys that give a nice profit and adds since I don't want to be stuck with an extra console to be honest.
But honestly my most regrettable buys for me are lots that contain potential great adds for the collection but are yet in my desired condition, I really regret those deals when cib probotector, cib shinobi III, cib ranger, cib runsaber cib etc etc were just not in my condition thanks to some cover damage wich barely reaches out of my standards, It's really nice that one can get extra money since the condition is plenty to many but in my opinion those are my most regrettable retro deals. It;s always a shame if a potential add is yet in the desired condition even though a deal is that great, however it is a blast if the most wanted games are in the desired condition and only less desirable items that one wanted are not or everything is in the right condition.
My most dissapointed retro deal was a huge gameboy lot, for most the standards are great and thus the profit was very nice even if I kept all the games that I wanted to keep but out of 15 potential cib gba adds wich included many uncommon great gba games some wich I still have yet to add in the collection only 1 cib game was in my condition
Still though as far as rarities or better games go, in allot of cases in my experience people play the bad games or the most mainstream ones wich leave the more uncommon rare and usually sought after ones in excellent shape left in the closet forgotten used one or very few times
that's one of the things that can favor one in allot of cases however it unfortunately is not always the case, but especially with excellent condition stuff when people barely use it it has a very high chance of those odds actually happening that one only play their mario all stars or sim city.