Same way you feel about Audi's I feel about BMW's. So personally I would never pay $10k for a 325 when I can get a 00-02 S4 for the same price if I look hard enough. But if thats what they typically go for, it looks like a clean car, the M3 wheels are nice, he claims that its been properly maintained...looks pretty good.
I would go test drive it, make sure the transmission feels solid, the suspension feels good, basically just look at it like you would any other car, but look at the maintenance records and also you can tell a lot from the owner. If hes an older guy with some money its probably a safe bet. You dont want some young BMW fanboi asshat who didnt take care of it, he just wanted to get all the fly honiez.
Someone on here knows more about them than me, so I know they will chime in sooner or later.