I almost bought a used 2012 535i with 7000 miles. The price was right & it was the right color & equipment. On the test drive I noticed a slight hesitation on acceleration coming out of a curve. The salesman said it was the turbo "spooling" up & was normal. He also showed me a clean Carfax with absolutely no issues. I'm about to sign the papers & the sales manager has to disclose to me that it was a "Trade Assist" vehicle (he pointed out that this is totally different from a dealer buy back but I still don't the difference). Anyway, to make a short story long, the paperwork showed that the vehicle had been in the shop at least once every 1000 miles for the same unsolved hesitation issue. I would not buy the car.
The moral to this is that it was all done "in-house" and absolutely none of this was shows up on Carfax.