One point that your write up doesn't take into account is that a car dealer can get more for a car, the same, car, in general, than a private seller can.

A car dealer can put a short or longer term warranty on a car that gives a buyer confidence.

If you are buying a used Honda from a Honda dealer who is reputable (and my guess is that is 99% of them are or they wouldn't remain franchised), as opposed to the local used car lot, you generally and on average are going to have better purchase security than buying a car from a stranger that does not come with any warranty or guarantee.

A car dealer also has (in theory) experienced salespeople who know how to drag the most money out of a buyer and exploit their weaknesses. You as a private seller don't. So you will (on average not the exception) not be able to get as much for the same car as the dealer will.

This is not to say that the idea presented by the OP won't work (because they appear to be adding a level of guarantee to the deal). Just that in the example you are giving there are reasons why "you buy a used car for 20k". Reasons that are separate from what you would get if you tried to sell the same car for 20k.