If you did something to the gun, especially if it was something that caused a safety issue, Id say yea. Just because the gun doesn't work when you shoot it, or isn't accurate when you shoot it, really doesn't mean anything.
Ive bought my share of used guns and have had the occasional issue here and there. I go into to knowing that might happen, and most of the time, its a learning experience on my end, and in a couple of ways. One, teaches you to pay better attention when you look at the gun, although just looking cant tell you everything. Second, you get to learn how to fix things, as long as it isn't something above your pay grade.
A little while back here I bought a nice Browning HP at a local shop. Looked to be close to spotless and NIB. Seems the previous owner thought he was a trigger tuner, and did some fiddling.
First time I dropped the slide on a loaded mag, it fired a round into the ground in front of me and the hammer followed the slide down. ***? So, dummy that I am, I thumbed back the hammer and gave it another go. That time, I got a 4 or 5 round burst out of it. That was that. Ended up replacing the sear and spring, and all is good. Easy enough job, and I learned a little more.
Next time I saw the dealer I got it from, I told him the next time he sees whoever brought it in to sell or trade, to kick them in the ass and tell them it was from me. I told him the gun was unsafe and what had happened. He just kind of looked at me and shrugged.
But, that's what happens when you buy used guns, and you need to understand that going in.
I also dont usually buy used guns that haven't been cleaned and/or are excessively cruddy. I usually ask them to run a patch down an obviously heavily cruddy bore, at least to try and see what Im getting into. Ive been burned a couple of times there, and learned my lesson.
I also wont offer or pay anything close to what they are asking either. If you cant bother to even try and clean something before you sell it, and its obvious that's how you treat things, what else can I expect? Im certainly not going to pay for it.