Well if you really want to be realistic, then
you could look around at what any used car is going for today, not just BMW's. You'll see that they're all taking a beating in terms of resale. I'm amazed at what I could move into today that I couldn't have moved into just a year or two ago. And I'm equally amazed at how little my pristine E46 M3 Comp is worth.
What's going on isn't unique to BMW's. It's not even unique to cars. The economy just plain sux right now.
As far as this guy's $750 Z3, I have to agree with those that think he got a good deal. Sure, he's going to have to put a little into it, but that's the fun (for some of us anyway) and he fully expects it's going to cost more than the initial outlay.