I am basing that on my meager knowledge of BMWs. The car should not go into failsafe mode for 'no reason'...there will be a fault code.
Dollars to donuts he will reset it and give it back to you. Try and get you to keep it until the dealer can blow you off. Dealer is paying this guy, right? Probably the shop that does all the work for the used car lot?
I'd bring it to a BMW dealer, spend the $$$ and get a real diagnosis: if the BMW dealer says "car is fine, we cant explain it", then you are as good as you can get. But if the BMW dealer sees issues, NOW is the time to press your case.
Penny wise, pound foolish?
Keep it as simple as possible...but no simpler.