The only remorse I have about stuff I've sold is for the things I let go too cheaply.
I had a 1964 blonde Rickenbacker 12-string that I sold to a close friend for $400 as a favor. He admitted years later that he'd held onto it less than one week, as he'd immediately re-sold it for $1200. When he told me this, it was in the context of how much money he'd "lost" in the transaction, as that guitar would go for $4-5K today.
I sold a 4-year-old synthesizer and Anvil case for $800 that I'd paid $4,500 for. I foolishly assumed it was rapidly losing value, as it was an analog-digital hybrid and all the new products coming out were all-digital. Today it'd go for around $4K again.
Those weren't the only episodes of financial stupidity, but I prefer not to think about all the others.