well..since we are also talking cars....
I regret selling the 1969 Chevelle SS396..I purchased new off the showroom..This was the High Performance model..It ran in the 11's right out of the showroom...I only had it for 6 months..around 5,000 miles (mostly racing miles)...How about the factory gave me a 5 year, 50,000 mile warranty..

The SS swallowed a valve...blew out 4th gear....and destroyed the rear...all covered under warranty...

As for regretting sales...There was a lot of keyboards I wish I kept..too many to mention...from Crumar stuff..to Jupiter 8's..
I even wished I kept a Farfisa G7..
