Almost all the handguns I own I've bought used. All my first rifles were used (though handed down from my grandfather, to my father, to me). A quality handgun will outlast the best car. The two are not similar in the least, there's only one thing you should do when purchasing either. Take a look under the hood, when buying a used gun make sure you break it down first and take a look to make sure everything is intact. If the seller won't let you do that then there's a good chance there's something wrong.
People sell guns for a number of reasons, I sold the two glocks I've owned because neither felt right to me. I got one, disliked it and sold it. A year later I kept hearing all the praise, and the newest generation was out so I picked up another and was again not pleased with it. Some people sell them because they didn't know what they were buying. I got my .454 from a guy who wanted a big 'badass' gun. Turns out the big bad gun kicked him in the head and he wanted nothing to do with it, his loss my gain. A lot of people will buy a revolver and upgrade to a semi later on, they'll sell the revolver to fund their new gun. Some people simply have to many guns and need to make room for new ones.