That is a good point, although you could still get it from a friend of a friend - ask your friends to let you know if they know anyone who's selling their car.
Also, you oculd buy it from a friend if you go into it with the understanding that you're not trusting him on this point, you're getting it checked out yourself, and the deal is final.
If I was selling a car, I would encourage the buyer to take it to a mechanic, but would also make it clear that once the guy buys it, it's his - period, no matter what happens. Otherwise, "ain ladavar sof."