I regret selling my CD32 and SX-1 Expansion a little but frankly other than the Akiko chip I can do everything else with other machines I own.
The one I most regret selling is my A3000 desktop. I sold it to buy an original Xbox. I had other Amiga's at the time and used my A2000 040 28Mhz more (which was also my first Amiga as I was late to the game).
I have since purchased another A3000, but I sometimes wonder where that original A3000 is now.
The biggest regret I have is non-amiga. I threw out a SX-64 in a move cause I literal had no space left in the van. I sure miss that box.