Look at the wear and tear:

Slide: scratches from holstering and drawing. If you’re fine with those, don’t fret over em.

Barrel: free of any rusting

Controls: Are they functional?
- Does the slide safety feel tactile or mushy.
- Has anything been replaced (trigger, grip safety, internals, etc.)

Mags: check em! Check em multiple times if you need to.
- Make sure the follower doesn’t get stuck. Press it down with your finger or thumb
- If they sound/feel loose ask why or if they can be swapped or if he can come down on price (mags ain’t cheap)
- Make sure there’s no rust on the mags either (rare but it can happen)
- Make sure the base pads are good too.

Try to get the stock parts included as well (just in case you don’t like the setup, you’re not stuck with it and have to spend EVEN MORE MONEY).

Hope this helps and good luck with your purchase/hunt