The vast majority of firearms I buy are new. However, I have no hesitation in buying used, if it is something I want, and appears lightly used, if at all used.
As an example, several years ago I purchased a Colt XSE stainless 45 "used", in the box with all papers, etc. I got it for a "used" price, but I'm here to tell you that gun was in better condition than a lot of "new" guns I've bought! I absolutely couldn't tell that it had ever been shot. Furthermore, it has proven to be an extremely accurate and reliable gun, and is right now my carry gun!
Another, is a M70 Classic stainless synthetic 270WSM I got about a year ago. Upon inspection, the rifle was absolutely brand new, it had never even had a scope mounted on it. But I got it for a "used" price.
Why and how these guns ended up for sale as used guns, I'll never know. A lot on possible scenarios can be imagined! But, I love it when I spot deals like those while I'm cruising the used gun racks!
Another example, is a used stainless 700 in 7mmSTW, one of my sons bought against my advice. He brought it to me to get it lined out. One of the first things I did was check how the stock fit. Upon removing the stock, there was a folded up piece of matchbook cover between the barrel and stock, so I knew someone had been trying to get it to shoot! The stock fit in the barrel channel was OK, however, the action literally rocked back and forth in the stock. To cut to the chase, after I had my gunsmith bed the rifle properly, the rifle is a tackdriver! What is even more amazing, I accidentally discovered who the individual was who had the rifle originally, who couldn't get it to shoot, when all it needed was to be properly bedded. He was properly humbled when I explained to him what a tackdriver that rifle turned out to be!
Another, is a M70 Classic stainless synthetic in 300 Weatherby, I bought used nearly 10 years ago. The rifle was used, but obviously barely used. I bought it, and that rifle turned out to be just about a 1/2" gun with factory loads! I never have bothered with handloads, it shoots factory loads so well! This rifle was probably sold or traded by the original owner because he wasn't liking the recoil level of a 300 Weatherby compared to the 243 he'd been shooting! There also was a rash of "used" 300 RUM's after that caliber came out as newest hot rod, and a bunch of people discovered they couldn't deal with the recoil!
So, there are a lot of reasons guns end up for sale on the used gun racks. Most probably aren't because the gun is a lemon, but for other more innocent reasons. And even some of the lemons can be turned into lemonade by a more experienced or determined shooter.