I only have two items that I have ever sold that I regret and they both have slightly different stories.
1. My 1928 4"x14" Ludwig Deluxe (Black Beauty) that I sold 14 years ago durring a time when my exwife and I were both out of work and had 4 kids to feed. The stipulation was that if the buyer ever decided to sell then I asked if he would give me right of first refusal. Long story short here, the guy that bought it ended out of work and called me last year and sold it back to me. My grandfather bought it brand new and gave it to my father who gave it to me. Was elated to get it back.
2. About 7 years ago I sold my fathers set of Rogers drums. It was 1964 Holiday series kit (12,13,14,16,22) in a pink champagne sparkle and a matching canister throne and Dynasonic snare. I was not a big fan of the Rogers drums even though they sounded good but I was living in an apartment at the time and space was not at a premium so I had to find one or two kits to go. Never regreted selling it until he passed away earlier this year. Sold it to a guy in Green Bay Wisconsin. If this rings any bells to anyone out there let me know, would love to see the kit again.