I have used KSL classifieds on 1 shotgun sale and 1 pistol swap. I sold an Ithaca 37 shotgun to some guy. No back ground checks or anything. I did check his ID and had him sign a bill of sale with his name, date, and the gun model and description.
The pistol swap was when I had a Glock 27 and swapped it for a Glock 23. I got the better end of that deal because the 23 I got had 2004 manufactured Tritium sights. The 23 also appeared to be brand new where my 27 was is good condition but over 10 years old. In this transaction we both ID'ed and wrote out the sale information to cover both sides of it.
Both times the guys looked pretty reputable and stable so I felt comfortable. If the guy wanting the gun seemed unsavory I personally would not do it. I know that might tick off some of you or seem very judgmental, but I just don't want to be the one who sells the "just out of the state hospital guy" his first pistol. I understand you can't tell in some cases, but if I get a feeling I am not going to sell it.
As far as buying, there are websites that can help you to know what to check for in a used semi-auto pistol, shotgun, revolver, rifle, etc. The handgun I got was in dang near perfect condition. Both magazines were so new they hadn't loosened up at all, the feed ramp had almost no traffic on it, etc. You could always try to get them to meet at a public range to test fire it once and make sure it all works but I haven't tried that myself yet.