That sounds pretty good then if you trust him. From the rail wear and the fact that it was his duty gun that he would have obviously trained with a lot I'd be weary of the 500 round claim, especially since as a cop he would know what guns are worth and wouldn't knowingly take a $(400) swing in price.

That being said, again if it feels better in your hands and you are comfortable with it, that is really all that matters in the end.

edit: also weird that if it was his carry weapon and he has discoloration and rail wear but the white printing on the gun seems to be in perfect shape, you'd think that'd wear down too going in/out and rubbing in a holster a lot.

edit 2: re-read that this is a guy on another forum, unless you have dealt with him before and trust him I'd be extra extra careful. That gun looks well-used, and compared to your Glock is probably roughly equal in resale value; not worth the risk IMO, unless you know and trust him. The discoloration noted as well as the condition of the trigger would raise a red flag even for a noob like me because that just doesn't look right, it's like looking to buy a used car and finding rust above a runner.