I've never sold any of my Amiga's and seem to keep buying more! (Pick up my 4th A1200 tomorrow!)
But I do regret selling my original C64

- I gave it to my stepsister when I got my A500, and when she was done with it, she dumped it and didn't offer it to me back! I was soooo pissed! Boxed......hundreds of games, mouse pack, new PSU....ARGH the pain!
I also regret getting rid of my entire obsolete computer collection back in 2001 while space and money was a big problem. I owned quite a lot of systems, but it was the add ons, software and manuals that is the biggest loss *sob*