I don't even consider cars at lots anymore! I have bought 3 from used car dealers but that is 3 out of 100s I have owned and bought and sold.
I see no reason. Unless they happen to have exactly what you want like a certain year Vette or certain color combination...you can ALWAYS get a better on a comparable one from a private owner.
Dealers are for convenience, easy financing, peace of mind (although it is a false sense) and ability to look and test driver several at same location.
Dealers and value or good price- DO NOT go along with each other.