Any regrets in selling gear?

Fortunately none…but the list of regret purchases is another story.

I guess it depends on your approach.
 

My approach is very deliberate and careful. Each purchase is intended to be a significant step forward and is preceded by lots of research and often auditioning. Over the years I developed rules like never invest less than 2x or more for the next component, and work towards a new system at an optimized higher level… so no weak links. Then I stay there for many years. With this strategy, what I leave behind is always far inferior to its predecessor. 
 

There are many other approaches like just swapping around… checking out different stuff. Etc. My objective is to get the best possible system I can afford… moving in a straight line towards it.

Selling my pair of Quad 57’s after 19 years of daily use. After twenty years I now have two pairs! The Quad 57’s are not the perfect speakers but what they do they do perfectly well!

I just remembered another item I regret selling- a mint Nakamichi BX-300 that had been serviced and had a factory update done. 

Sure, however it opened opportunities to try others. As some have indicated here, after they cycle through a bunch of stuff over the years or decades - sometimes , they go back and repurchase that lost piece of gear to tray again to see if its as good as they recalled the first go-around. Sometimes it is, and other times it might not be.