I have donater's remorse. I had a Premier XPK kit that I bought new in the mid-90's. I left it at a friend's house where we were rehearsing (in gig bags), and someone there dropped the bass drum, and it damaged the reso-side edge a bit. It still sounded fine, but I don't want to play on damaged gear, you know? I put it up for sale, didn't get any bites, so I donated it to a school where a friend teaches music. I got a brand new Stage Custom to "replace" it as my rehearsal kit, but quickly found out that the Stage Customs are not quite in the same league. Wish I could have it back, it was a great-sounding kit. But at least I know the kids are getting some use out of it.
I don't get seller's remorse too much these days, because I'm a little smarter about appreciating when I have something special (to me at least), and when I have something ordinary. I'll sell of the ordinary stuff and keep the special. Like the Fender Jazz Bass Plus that I spent a whole summer paying for on layaway when I was 18 years old. I mean, the Jazz Bass Plus was not the pinnacle of Fender's achievement, but I worked my ass off for that bass and I'm never selling it.
When I get buyer's remorse, it's usually a sign that I made a great choice and bought some really good stuff. Buyer's remorse often disappears the first time I play a gig with the new gear.