Needing a Cam Sensor for a Beetle?

I am looking at a car on Craigslist to purchase. Its a '99 Beetle. The ad says the throttle body needs to be replaced. I called about a new one. Even if it is just the sensor, a new one will cost $644.00. So upon calling back the guy says he has a friend coming to look at it but the throttle body is disconnected and runs fine but low RPMs. Now he says his friend looked at it (a mechanic) and says it is a cam sensor instead. I cannot afford to buy a car and find out I am being ripped off, but really want a Beetle. At $2,800 I cant decide if this is a good buy. What do I need to look for in case he is BSing me?