Well, its an excellent price. I would be cautiously optimistic. First, get the Carfax for any supsicious information (not perfect but worthwhile). Then, have an independent inspection done by a local BMW dealership (this will cost $100 or so) and then talk to the person who did the inspection. Try to find out who was the previous owner and talk to him/her if possible. Try to get a BMW dealer to run the VIN for the service history.
Then, go and check it out yourself being careful to look for signs of possible body work. Test drive hard and look for clutch slippage under hard acceleration.
If all checks out, go for it. I still would pay the extra for a CPO, but there are some aftermarket warrantys out there.