If the price of any used Glock starts with a "4", it's probably overpriced.
On the other hand, any Glock priced under $400 is probably ragged-out, stolen, stippled, missing mags, and otherwise undesirable (and that's putting aside the fact that sub $400 Glocks are pretty rare, even the on-line police trades in, which are usually 40's and in which shipping and transfer puts them back over $400.)
Most of the G's I've stumbled into in the last 10 years, have run me right at $400, and no way/no how was i getting them any cheaper. $400-$450 is a deal, if it's clean, and has the factory mags and stuff.