Trick question.
And the only real protection you have is that a dealer will have to convey a clear title to you or face losing his license. Other than that, they can sell you the same hidden damage junk any private guy can. Because where else do they source these cars? Same private people you can.
Best case the dealer adds value to the transaction by sourcing good cars which he then resells for a fair price. Worst case the dealer is a crook who rips off the previous owner and scams the new owner. Examples of both cases exist which is why it is always buyer beware and do your research.
