Tips for buying a used gun would be enough to fill an entire thread, lots of info out there! There are a lot of great buys to be had out there if you know what to look for. For instance, on a revolver you have to do a lot of things but first check to see if the gun is loaded.
You've got to get over your first impressions when you see the gun's finish. A gun with a pristine finish may be a gutted wreck, or the gun that looks like 20 miles of bad road may shoot like a house on fire. The outward appearance may give some indication of how well the gun was maintained, or it could just show honest holster wear. On a gun (of any type--rifle, shotgun, pistol) with adjustable sights you should look for burred out screws, loose fit, damage to the sights, etc. Look at the grips, the muzzle's crown, trigger, cylinder latch and any other external features. One red flag I always look for is a "burred" or damaged screw head on the gun's frame. This means that someone has been taking the gun apart--usually with the wrong size screwdriver--and doing who knows what to the guns internal parts. A 'shade tree' gun smith may have been monkeying around in there.
Next, look at the cylinder crane where it fits against the frame while the cylinder is closed. If there is a gap it probably means the previous owner watched way too many movies in which an actor dramatically 'flicks' or 'snaps' the cylinder shut with a flick of the wrist. I personally detest seeing this behavior and it's pure-dee ignorance. This can warp the crane (the part that supports the cylinder when the gun is open) and cause the cylinder's chambers not to align with the bore properly. If the crane fit looks tight, it's time to move to the interior. Open the cylinder, note the movement of the cylinder latch. You want a smooth release, but you don't want the cylinder to just flop out. There are several designs for keeping a revolver cylinder in place and if they're worn or damaged the cylinder may not held securely. Try the ejector rod and extractor...if they're bent or damaged they'll have to be repaired. Look at the chambers for pitting or other damage. While the cylinder is open check the bore as well. Look for pitting (rust spots that have eaten into the metal) and 'bulges.' A barrel bulge indicates that the gun was fired with an obstruction in the barrel, usually a bullet from a 'squib' or underpowered load that didn't entirely leave the barrel.
Close the cylinder and check to see if it locks up firmly. Don't look for just rotational play, clockwise or counter clockwise, but look for front-to-back movement as well. Next it's time to try the action. Hold the cylinder with light pressure between the thumb and forefinger of the left hand while you pull back the hammer with the right. When the hammer reaches it's fully rearward position the cylinder should align a chamber with the bore despite the light pressure you applied. Now with some models the cylinder will not lock entirely until the trigger is pulled--especially in older Colt revolvers. Holding the hammer to prevent it from falling forward, pull the trigger and ease the hammer down. If the action is "in time" the cylinder will align each chamber with the bore without trouble.
Now, you need to check the hammer/sear engagement. Sometimes the 'shade tree' gun smith will take it upon himself to "polish" or "hone" the contact surfaces between the innards of a gun to "smooth it out". If not done correctly, the gun won't function safely. So, this time cock the hammer back to full cock. With the hammer back, try to push it forward with your thumb without touching the trigger. If the hammer drops the innards have problems, time to pass on the gun. On a double action revolver you should also take the time to try the DA action, placing gentle pressure on the cylinder to see if the gun locks up correctly in DA mode and just to try the overall "feel" for the DA trigger pull. Personally, I'll pass on an 18lb trigger pull.
All that and I've only covered the bare basics of looking at a used revolver...lots more info to go, but it is out there on the 'net. Just Google "how to buy a used ______" fill in the blank--semi auto pistol, rifle, shotgun, etc. Here are some links about buying used guns from Chuck Hawks:http://www.chuckhawks.com/used_handgun.htmhttp://www.chuckhawks.com/used_rifle.htmhttp://www.chuckhawks.com/used_shotgun.htm