Howdy DocBoss, I woild like to welcome you to the forum.
Aside from the good points that Richard has already made, just look at the slide fit. Is it tight side to side? If they will NOT disassemble, pull the slide back as far as it will go and hold it back tightly in your hand. Now look at the rails. Is there excessive wear, deep gouges, any cracks showing? Frame Cracks usually show somewhere near the slide rails or dustcover (under the recoil spring), so inspect these areas well.
Check the ejection port for any alteration. Look at firing pin and firing pin hole for "truing" or enlarging of hole. Look at bolt face for scratches, gouges, corrosion.
Use your finger to gently tug on extrator for tension (good) and inspect for abuse. Push on ejector, to make sure it's not loose.
Look slide over for hairline cracks; especially at back of slide and underneath when you pull slide back and hold it back. Look close at that flat area that cocks the hammer and the rails in the inside at the rear.
Inspect the bore of the barrel. A gun dealer will have a bore light. Pull slide back enough to slip the light in the chamber and look down the bore. You do not want to see any rust pitting/corrosion. Dirty, and copper fouling is acceptable, but use that to your advantage in negotiations.
If a bore light is not available, hold the slide back and stick one of your fingers in the bolt area so that your fingernail faces the chamber. Hold the bore directly under a light source (usually the flourescent lights) and you can see fairly well.
Pull the mag, inspect the mag. Is it beat up? If so, my intenae go up. Check the feed lips, press the floor plate for tension. Now look at the mag chute on the gun. Look inside well. Look for scratches, look for any rust, dirt in the mainspring. Press on mag release with thumb. It should be tight. Slide mag in to test for a solid hold.
It is VERY poor etiquette to pull triggers and snap the firing pins on anyone else's gun without asking for permission first. If you are a serious buyer, it is usually not a problem. By all means do so. All R9's should have a nice long smooth as butter trigger.
With trigger pulled half way inspect hammer and firing pin area from the back. Look for "peening - bad", on both hammer face and where it strikes the firing pin.
I'm sure I missed something here. It is very late. Others may add to this list.
It took 4 times longer to type this 2 times longer to read this than it will to do the actual inspection. So please don't be intimidated.
Also, it is can be insulting to bring a set of Visi-Visors (Jewlers/Gunsmith magnified visors), but a neat tirck is to bring a pair of extra strong reading glasses, They really help in finding cracks and imperfections and such. The seller will never be the wiser.
Good Luck
