Any regrets in selling gear?

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.

A few items that are pricy to replace now (some would require refurbishing).

Roger's LS3/5a's purchased in 1978 for $500 + the cost ($130 ?) of a 1-way plane ticket from London to LAX for an X-Girlfriend.

Nakamichi 700ZXE that I paid $75 for in perfect condition and sold 20 years ago for $600, or so, as we recently unearthed two large boxes of my wife's cassette tapes.

I do miss my  vintage Dynaco and Mac tube gear, but much prefer DH/SET to them (mainly miss the PAS preamps when running tape/line through them - phono was so-so).

DeKay

Oh yeah. Bought and sold lots; some I re-bought later. Many components I regret selling; others I sold off and never really desired again (learning experiences). Then a few times I bought/sold the same damn component 3 times (or more) - that’s generally a good indication I like many things it does, but can’t live with it long term. The audiophile equivalent of a toxic relationship lol.

Many components only shine when used in conjunction with the right partners and right system. I’ve had huge swings in opinion on a piece due to this. So that’s my justification to keep a few (or several) of each component on hand at a given time lol. And also in being hesitant to let go, in general. 

Some components get harder to find as time goes on - as interest wanes it becomes forgotten and drops out of general trade (they’ll probably hit circulation again when the last owner dies and it hits estate sales in the next 10 - 20 years). But generally, most lose value in the long run. The exception to this was some of the really esoteric headphones in the 2000s. I definitely regret selling many of these for "too low" prices back then. I mean like Sony MDR-R10, Sony Qualia 010, Audio Technica ATH-L3000, Grado HP-2...

I sold my Conrad Johnson MF2550SE amp because I had new CJ tube amps.  Make no mistake, the tube amps are superb, but I have wished over and over that I didn't sell the solid state amp.  It was spectacular.

I was thrilled to get rid of my Musical Fidelity CD, which everybody loved, other than me.  But, it doesn't make up for selling the CJ amp.

@dekay "...but much prefer DH/SET to them (mainly miss the PAS preamps when running tape/line through them - phono was so-so)."

I still have one DH preamp and let go of all of my higher power DH/Cary and lower power DH/Inspire amps. Came close to Dennis building  me a pair of KT88/6550 monos but the build-deal fell apart.  Which DH/SET do you still have?