I threw away a Sega-CD front loader because of what was probably just a blown fuse (didn't know that at the time). I replaced it with a used top loader which has been totally reliable, but the front loader was way more cool.
I purged some game boxes to make space, but not all of them. Instead of throwing them out, maybe I could have flattened them. At least I kept the Ultima boxes.
I'm on pins and needles about some Sound Blaster cards that would now be very expensive to replace. I especially wish I'd bought a duplicate of the OPL2 SBPro, which was my first Sound Blaster card. I'm fond of how it sounds in DOS games, because that's what I had back then. I wish I'd bought duplicates of it back when that stuff was cheap.
There was a time that I remember these cards were so cheap people considered them as donors for OPL2 chips.
Future regret? I have 3 CRT TVs for use with game consoles, one of which I especially like, the other 2 are okay. Depending how they hold up, I *might* start to regret not buying 1 or 2 more that I passed on. But they're bulky, so hoarding them is difficult. Yet it can be argued they are the most deserving case to hoard. The tubes wear with age, they will start dying, and nobody outside of niche interest wants to store them so healthy examples will thin out quickly.
But then there's some non-regrets. I've kept records of old eBay purchases, and there are some jaw droppers in there. Prices have changed a lot over the years. And while I cringe at what I sold some things for, I can't keep everything.