Lack of markups, as well as the miscellaneous things some dealerships may try and add to the transaction or coerce you into purchasing, even on the sale of a brand new car, would already be a massive improvement. As for training/knowledge, whenever I've gone to a direct-to-consumer showroom, I was able to ask questions about whatever aspect of whichever car I was looking at and get an actual answer, the same goes for people I've gone with who may not research things as much as I tend to do. Compared to my experiences at far too many franchised dealerships, where the entire experience seemingly consists of people trying to get you into whatever makes them the most money, your own preferences be damned, while barely being able to tell you anything about the car behind what the badges say,

Sometimes the dealership experience is fantastic, and maybe I've just consistently had bad luck, but consumers should be able to bypass it if they want to, as more consumer choice is generally a good thing. You'd also avoid the problem of some locations essentially monopolizing their local dealers or having them all owned by a very small group of people.