Used pistols are easy to sell, so you'll always get your money out of it if you need to. Around here, that pistol with the extra magazines (clips) would go pretty easily to a private buyer for around $400.00, though a gun shop would insult ya if you took it to them looking to sell it. See if he has any accessories (holsters, lasers, lights) or a boat-load of ammo to go with it.

Guns wear out just like any other mechanical critter. The frame (the part you hold on to) on the Glocks is made out of polymer while the slide (the part on top that moves back and forth length-wise), along with the barrel and internal gizmos are all steel of varying types.

Signs of wear are shiny spots where the moving parts engage each other or on the outside of the slide along the edges.

If the feller took care of it, then the inside should be clean. No gunk around the breech (where the slide pushes the ammo cartridge into the back of the barrel and kicks out the empty casing after firing), and no grit in the internal workings. The inside of the barrel should also be clean and shiny. "Gunk" and "grit" in a gun comes from mostly fine brass and/or copper shavings (scraped from the ammo cartridges during the firing process), unburned gun powder and carbon. Gun shops and retail stores all sell the cleaning supplies to keep any firearm in top-notch shape and it takes all of 3-5 minutes to completely scrub a pistol.

Ask the feller to show you how to field strip (pull it apart for cleaning) the pistol for you. If he knows anything about the gun, it'll take him less than 20 seconds and requires no tools. Once it's apart, you can look for any gunk or grit plus look inside the barrel (it'll be apart from the gun by then. No chance of shooting yourself in the eye!). Just hold it up to a light source and look inside. There you will find grooves machined into the inside of the barrel in a spiral pattern, called the rifling. The rifling puts a spin on the bullet as it passes down the barrel and stabilizes it for it's flight to the target, much like how a quarterback put a spin on the ball when he passes it. The spin increases it's accuracy and range. If the pistol has been neglected, these rifling grooves will show copper fouling packed into them.

If it looks like it's been well maintained and it's something you are interested in. Consider jumping on it then take it to a qualified shooting instructor to learn how to use it safely.