I prescribe to the "I don't sell guns" method... most of the time.
I have sold some guns in the past that I'm happy to see gone. I will never own another Armalite AR, as I had an M15 that was HORRIBLE. FCG pins would walk, the front handguard shroud broke loose, and the buffer tube was SOOOO rough from machine marks that you literally could hear the buffer spring scratching over the muzzle report. Sold it to a neighbor, losing money on accessories but I was happy to see it go.
I sold a Saiga 223 that was horribly inaccurate. I couldn't stand that rifle. It would go bang no matter what, but shooting less than 3" groups at a hundred yards was not going to happen. 4+" groups if you didn't have the right ammo.
I sold a few that I hated to see go. It was post 2008 and there was a run on ammo. Prices spiked, and I was shocked that I couldn't find WWB in .45acp anymore. If I could, I would have to pay $35.00 per hundred. While that's a normal price now, I was used to abour .22 cents per round back in the day. So I sold some 1911's and just went with 9mm firearms to save money... which was tight at the time. I wish I could go back and undo that... but I'm not so sure I would have been able to scrape by either way. It was also when they were almost giving four wheel drive trucks away, and I sold a 6 year old truck with 60k miles on it for less than 8 grand. Those were some tough times, and I had to make do. Would have never sold them if that wasn't the case.