Note that BJ doesn't write the copy that appears in the lot descriptions. It is supplied by the seller.
My experience at Barrett Jackson, both from seller (charity car) and buyer (my Cobra) was excellent. Everyone talks about the "anecdotal" evidence of tampering, but it's always "I heard from someone that knows someone else that says someone did something". So far noone has ever come forward and admitted doing something and over all the years, if it happened, someone would have squealed by now.
The only even that comes close is the buyer that balked on the Bugatti. I was in the skybox watching when Craig Jackson paused between cars and told how the buyer was "overzealous" (he couldn't afford it and though someone would outbid him, and was banned from all BJ auctions in the future) so asked if the previous bidder would still want it, and he didn't so Craig bought the car. It looks like it is a "company car" though since it always has a BJ wrap on it.
Commissions are something else, BJ is 8% seller 10% buyer, more if the car has reserve. I think Mecum is the same or more depending on venue.