I like Timo answer! Ditto...I regret giving equipmemt to "friends" and not getting paid what little I was asking! I do perhaps regret trading in an old pr. of Fisher XP-7's for some stupid spks. in the 70's.
Speakers you regret selling
I was lucky to own Sonus Faber Extrema, Acoustic Energy AE1 Signature, and Dynaudio Confidence 5 in the past. They are rare in used market and can make any owner happy for the rest of his/her life. So I swore to myself I would never sell my Sonus Faber Amati this time though my wife has heard that a million times. What did you regret selling?