Even if the P99 is practically new in the box with all its original trappings, you should at least field-strip it before purchase and examine it inside carefully. With today's hard finishes, a gun can see a lot of use without obviously showing it. The more edge wear or nicks and dings it shows, the more searching should be the internal examination.
If the pistol passes muster on mechanical condition, THEN go to cosmetic condition. I'm always amazed at the number of buyers who scrutinize for any scratch and nick in the finish, but just ASSUME the mechanical condition inside is commensurate. Bad practice.
The first condition issue is: Is the gun mechanically sound? Is anything broken, cracked, missing, or buggered up in some way? Look for signs of hard use, or abuse. Is there heavy peening or deformation of the surfaces that absorb recoil impact or stress with every firing cycle? Are any metal-to-metal contact surfaces flanged or excessively burred from wear? Inspect the rear vertical surface of the "chin" of the slide. Look for anomalies. Any obvious sign of alteration or kitchen table-gunsmithing (especially Dremeling) is, in my opinion, grounds for rejection.
Don't fret too much if the gun is dirty; actually it's easier to spot problems and wear patterns than if the gun were clean. A lack of cleaning does not necessarily reflect abuse, but copious scratching of the plastic on the left side of the frame (where it gets negligently tossed on a table), or burring up of the slide serrations shows a lack of care in how the gun was handled. Examine the plastic frame and subassemblies for cracks, especially inside the trigger guard, behind the trigger and adjacent to the locking block. Look carefully at the sliding sheet-metal parts of the trigger mechanism and the corresponding cam surfaces cut in the slide; the contact surfaces should be reasonably smooth, and bright; straight where they should be straight and curved where they should be curved. If you see signs of hand-filing or bending, or irregular wear, beware.
Is the bolt face in the slide smooth, without ringing or pitting, and without a puckered, oversized or ragged firing pin hole? With the firing pin safety block depressed, push in the rear end of the firing pin (noting if it moves smoothly against its spring) and examine the tip protruding from the bolt face. Is the tip of the firing pin smooth and hemispherical, or rough, pitted or undersized compared to the diameter of the hole? (And if you can't see the tip, it's broken off.) When you let up on the firing pin safety block, and release the firing pin, does the block positively re-engage? The pin should then be locked.
Is the extractor claw chipped? Does the extractor pivot freely under thumb pressure? Is either of the sights mashed? Are the slide stop engagement surfaces deformed?
Examine the feed rib on the underside of the slide; is it ragged or chewed up? Examine the sear surfaces of the striker mechanism: are they free of serious burrs?
Pull out the barrel for separate inspection. Is the chamber bright and shiny and without scoring, pitting or other visible defects? If it isn't you'll have extraction problems. Reject any gun with even a shadow of a ring in the bore. Look VERY carefully at the horizontal locking surfaces of the barrel and slide; they will likely be bright where they have seated and mate against each other, but neither should be deeply indented or burred. Wipe any oil or schmutz off the underlug cam surfaces of the barrel and the locking block. Examine them both under good light; if you see any cracks or serious upsetting of metal, or if the locking block is loose in the frame, pass.
Reassemble the gun and check for function. Let the slide snap forward and dry fire it a few times. Does the sear always engage, or does the striker follow the slide down? Pull the trigger to see. Will pulling the slide set the trigger into SA mode? Will it reset by itself after slowly releasing the trigger and retracting the slide? Does the decocker work?
At the same time you can check the trigger pull: reasonably smooth, or crusty and ragged? Dangerously light?
Are the magazines good, lips straight and unmangled? Look for cracks in the corners of the lips, especially at the top of the magazine's spine. If you find plier chew marks on the mag lips or dents in the sidewall or a heavily scuffed floorplate (from being repeatedly dropped), deduct the cost of a new magazine. Try both mags in the gun to make sure they fit and latch; pull back the slide and make sure the hold-open works.
Now for originality: Do the numbers match, especially the barrel, which is the most commonly replaced part? An un-numbered or mismatched barrel is a replacement.
Others may disagree, but I personally steer well away from any used gun of recent or current production, such as a P99, that has been refinished, modified, customized, or festooned with aftermarket parts, unless I know who did it, and WHY. There's too great a possibility of latent problems, which may explain why the gun is for sale.
M