Have always struggled with these thoughts. I've had sellers regrets many times. With the Taipan Vet 1 I think i bought and sold a .22 compact three times over. Done the same with a Cricket II - the first one was an OMG amazing laser with the H&N 21gr pellets. Sold it and then went through about 3 more Crickets never finding the same accuracy again. And there have been others too.
I have a few airguns, that like you get very little use, but I know not to sell because finding another gem like it may never happen. A Cricket Carbine Mini in .22 is one of them. Always first shot accurate and always tiny groups out to 50 yards. After the Vet 1 back and forth I know this one stays in the safe and comes out once every month or two for a days worth of shooting.
I have more than enough airguns so I try to subscribe to the sell one to buy one. For a while I would sell the one before buying the other. Not that I needed the money to do it, I just though it was proper protocol. But after too many sellers regrets I moved in the direction of buy the new gun and make sure it satisfies, then sell the other gun.
My only planned purchase this year is a .25 RTI Mora Standard and I will definitely run it through it's paces before I consider selling whatever it will be replacing.
