... any suggestions on what to avoid when looking at a used firearm...
Taylor, take off the barrel and examine the bore by looking toward a light or the sky. Move the barrel around until you have a clear, glare free view. If the barrel has not been cleaned, ask the vendor to clean it. Look for dark spots (pits) or streaks (leading or scratches). A high use gun will show a lot of streaking (leading) in front of the chamber. A pitted barrel is to be avoided, as is a scratched one. Many modern semis have chrome lined bores, so you should expect near perfection on a low usage gun.
Run your fingers up and down the length of the barrels feeling for dents. Many small dents can be hard to see, but you can feel them. Examine the rib as well.
Take out the choke tube and examine the threads. Cross threaded or damaged threads are not good.
With the gun still apart, now look at the bolt face. A nice round firing pin should be visible in a neat round hole. An oblong or distorted hole is to be avoided. Any gun that has been fired to any extent will show an imprint of the shell on the bolt face. Greater usage will show a stronger imprint, high usage guns will show slight pitting or a roughness to the bolt face.
Reassemble the gun and pull back on the bolt. It should feel fairly smooth, not gritty. The spring tension should be reasonably stiff and consistent. Ask for (or bring with you) a snap cap. Insert the snap cap in the chamber, release the bolt, turn the safety ON and pull the trigger. The gun should not go click. Then release the safety and pull the trigger - it should break crisply without undue effort, and the gun should click with some authority. Pull back on the bolt smartly and the snap cap should eject cleanly.
If the barrel bore is in pristine condition and the bolt face looks pretty new, then you have a low milage gun. If the barrel is full of leading and the action feels loose and kind of rough then it has been shot a lot.
While you are at it, stick your finger in the loading gate and push forward on the magazine follower, it should slide freely. Feeding problems on a quality gun are quite rare, but look for any obvious bent or sticky parts.
Guns such as the Beretta 391 series should function flawlessly for tens of thousands of rounds if cared for properly and cleaned regularly. In the end springs can weaken or break, but these are easy to replace. It is genuinely difficult to wear the gun out completely, abuse or misuse is what you are really looking for. I am sure that others will have many tips or hints to add, but this should give you the basics and ensure that you find a used, not abused gun.
Sharptail