Years ago, I had a Gateway P100 Win95B system that I was keeping at my girlfriends house. Her basement flooded and the computer was submerged for at least 48 hours. After we cleaned out the basement, the computer was thrown away. At the time, I was under the mistaken belief that a waterlogged computer would never be good again.
A few years later, I had a chat with Chris over at BadCaps.net and he told me that computers are surprisingly resilient if they're thoroughly dried. In fact, he gave one of my KT7As a bath & scrub before recapping it. That KT7A is now one of my favorite PCs and it works flawlessly.
Nowadays, I kick myself in the ass when I think of that old Gateway P100. It was a really nice system that would probably have been OK if I had just rinsed it off and dried it out.
I also sold a boxed mint copy of "D" that I now regret. My brother tells me that it's somewhat collectible now.
I'm actually about to sell an item that I may regret later: a MS Explorer Trackball. I had two of these that I purchased back around 2005. While I love the way the body of it feels, I've always thought that the action of the ball itself was horrible. There were plastic lips on the "ball cup" that were supposed to hold the ball in place, but actually just increased drag on it. The metal bearings inside the cup were awful too and always gave me a "fingernails on chalkboard" sensation. I used one of the two trackballs for several years but had to modify it to get it functioning satisfactorily. I removed the plastic lips on the cup and frequently lubed the bearings with oil or hand lotion. It was tolerable that way. However, for an optical trackball it required just as much maintenance/cleaning as a regular old mechanical trackball.
Even though I removed the second trackball from its package, it is still essentially brand new. This is the one I'm thinking of selling since they're worth a few bucks on eBay. I'm just afraid I'll regret it later if I ever develop really serious RSI. Aside from the Logitech FX and the MS Explorer, there weren't any truly ergonomic trackballs ever made. Even the FX and Explorer aren't perfect and have their drawbacks. However, If I sell this thing and change my mind later, I will be S-O-L.
There are some things I regret buying:
Matrox Mystique (I quickly sold this POS and got a Millenium + Voodoo 1 instead.)
Gravis Xterminator Gamepad (They came so close to perfection, but it's just so terribly flawed.)
Radeon 9800XT (Twitchy drivers. Crap for Win9x.)
BattleCruiser 3000AD (Nothing else to say, really.)
GeForce 4 Ti4800SE (I got fooled by the "Ti4800" part of "Ti4800SE". The "SE" stands for "Shiat Edition".)