I was thinking in buying a glock 21 for $500 used with 3 hi-cap mags. It's in good condition. I don't know much about them. I would like to know what to look for and if its a good deal?

Thanks

Not an especially good deal, but if its like new then get it.
Look for slide peening where the locking block hits it.
If its severe, its been shot a lot.
Rust, barrel bulge, cracks, the usual stuff.

Welcome to the boards!

What generation is it?
How many rounds have been through it?

Unless it's a Gen 3 with only 100 or so rounds through it I don't think $500 is a good deal.  It's an okay deal at best but I can pick up a NIB G21 for $499 at my local shop here in Virginia Beach.

Wait, I just saw you are from NY so maybe this is a pretty good deal...?  Esp with 3 "normal capacity" mags.  Find out a little more about the condition first.

BTW - I love the G21 even though it's a big handgun!

$500 is expensive.  I have seen some 3rd generation G21s with night sights going for $400-425 on the EE.