I owned one for three years. Bought it second hand from a fellow who hadn't shot it very much. Got the gun and two magazines for $400. I bought two more magazines and a carry holster, and put about 7000 rounds through it over those three years, and then sold it for that same $400 (though I did include all four magazines and the holster) that I initially paid for it.
How used is it? (i.e. How many rounds, roughly, has the owner put through it?)
How is the finish?
Two mags, or more? (This is a big consideration, actually, because those things can be hard to find and I don't want an auto with fewer than four mags.)
Any "bonus" items? (holster, hard-side case, etc.)
If you're just talking about the gun and two mags, I'd start at $350 and see what he says. If he's a really good buddy, I'd offer him $375 and walk off with it.