1 piece of audio gear I really regret selling is..

Wadia 27ix ... thought the company was going under so I sold it dirt cheap before I took too big of a hit.

Also, Well-Tempered turntable... I don't have anything right now to play the thousands of albums I have.

Original Keith Monks RCM. Thought it was too tweaky at the time. We were moving around then and every time we did, something would go amiss. Wish I could set the Wayback Machine to the early '90s

Proac Studio 250 speakers. Best I ever had on my modest budget. But I got bit by the bug and tried to improve. Big mistake

My Mirage M760's What a dope I was.
I sold them and bought Defenitives BP10's.

two pieces come to mind--first is my b&k avr-202, which had fantastic build quality, sounded great and had the best learning remote i've ever used. i "upgraded" to a more heavily featured denon, which sounds like shit in comparison.
the second is a cambridge d-500se cdp, which was an essentially flawless player i sold on impulse. i've not been able to replicate it at anywhere near the price.