That G17 Gen2 probably was a LE trade-in with lots of holster wear. Which might be why the guy had the slide chromed.
I bought a G22 Gen2 LE trade-in a few years ago for $300 and the slide was worn to the white near the muzzle. Otherwise it's a fine pistol and these Gen2 trade-ins are probably worth around $350 or so. Chroming the slide is a waste of money and as others pointed out could affect reliability. I'd pass on that one.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek