I would recommend you buy a copy of Fjestad's Blue Book of Gun Values. It lists about every gun ever made and gives prices based on condition.
Unfortunately, condition is mostly a cosmetic thing. I guess they assume "if a gun looks good it must be good mechanically". Of course that isn't always true. I have seen police trade-ins that spent a lot of time in holsters and looked really bad. Mechanically they were very good and probably hadn't been shot much. On the other hand, I bought a beautiful S&W mod 27 that looked like new. When I got it home, I found the cylinder was bad and a few internal parts had been damaged beyond repair by some bozo trying to do a trigger job. I had to send the gun back to S&W for repairs.
For the novice, I would stay with brand name guns such as Ruger or Smith & Wesson. That way if you happen to get a bad one, their warranty will take care of most repairs.
As for specific things to look for... so much depends on the type of gun you want. Double action revolvers and single action revolvers all have their quirks. In a 357 DA revolver, Ruger GP-100 or S&W 686 are very good. In a 357 SA revolver, I would recommend a Ruger Blackhawk.
It's not unusual for a used gun to bring new or near new prices. If you really don't know much about guns, it would be wise to buy a new one. Even us olt timers get burned now and then.
