Buying used guns is almost an "art" because there are many out there that to the untrained eye and inexperienced hands are "great" - NOT! It takes knowledge, experience and some mechanical ability to be able to look at a gun at a show, LGS, or someones house and in just a few minutes determine weather or not to pull your wallet.
I would strongly suggest that for the best possible chance of getting a quality piece you should either learn exactly what to look and check for or at least bring someone else along who really knows their stuff. If not, you are at the mercy of the seller and sheer luck - I never liked those odds......
Be very suspicious of something that looks to good to be true. Learn how sellers use certain words, or phrases to leave plausible deniability when selling something. Phrases like " AFAIK, SEEMS to be, APPEARS to be, LOOKS like, MINTY, I am no expert BUT....... etc. etc. If when using these phrases in an add or even in person they are found out or seen through they take a step back an say well I thought, or well I am not a professional - so on and so forth. Like I said, it's sort of an art to wade through all the BS stories. I suppose there are SOME legitimate guys who truly sell things that are not right and they have no idea, but I'd think most who are selling something know exactly why there are selling it - so be careful out there, learn about what you are looking to buy and it's OK to be suspicious. Knowledge is a great thing!
Not making fun here or trying to insult you, but after buying 4 out of 5 "lemons" I think it's time to LEARN and check out prior to pulling your wallet or hitting the buy button - just saying......... There are many beaters that can technically be restored and repaired but in most cases it pays to spend a few dollars more and get one without issues. Unless you can determine exactly what the issue is on the spot, know exactly how to fix it, have the parts or know that they are readily available and not expensive - it may be better to take a pass and continue the quest. There is always another one down the road!