Every car on the auction block is there for a reason
Your used car manager buys a car he hasn't inspected, pays a premium fee to bid on it, buys auction insurance in case it's a problem, and pays a transporter to haul it home. He does all that in a room full of other dealers bidding the price up. And the car is on the block in the first place because the dealer who had it couldn't retail it. You're paying top dollar, sight unseen, for someone else's leftover.
Off-market sourcing
Dealers went looking off the street, but scraping Marketplace and Craigslist for cars that are already publicly listed just moves the swarm online. You're still bidding against every other dealer, on a car whose owner is already shopping it around.
The bid-ask spread closes fast when the seller knows what they have and how many people are asking.