The first thing i do when purchasing a used gun is do some research on the firearm, i check reviews for reliability, accuracy, availability of replacement parts, magazines and accessories and average price range. I do this before i even start looking around the gunshops.
I personally do not purchase firearms that i cannot physically inspect, i do not buy online or by mail. I normally call around to a few local gun shops in my area and ask them if they have whatever firearm i am looking for. Then im off to the gun shop to check it out.
First I make sure the firearm is unloaded, Then i look at the overall appearance of the firearm, make sure you dont see any cracks, bulges, rust, or dents. Make sure that everything seems to fit tightly and nothing is loose or crooked ( some guns are designed and built with loose tolerances , so be sure to do your research ) Make sure there are no missing screws or pins, if your not sure just ask the seller.
Next open the action and inspect inside the firearm, again checking for any unusual looking wear or cracks, Look to see how clean this used gun is, this will give you an idea of how well it was taken care of. Check inside the barrel for any scratches or bulges ( ask for a bore light to see inside the barrel ). Work the action a few times to assure smooth operation and if you are able to, dry fire the firearm to make sure of a smooth trigger pull. If the firearm is magazine fed be sure that it locks in properly and releases smoothly.
If i decide to purchase the used gun i will often negotiate price, Used guns are like used cars, point out a small scratch or a loose grip and ask the dealer to lower the price or throw in a box or two of ammo, As long as you negotiate respectfully you will often get some very good deals !