The online dealer changes randomly and the used car dealer changes after 3 races.
Online dealer changes every time a new set of seasonal events shows up. So far at least.
And to add to your other point, 6 of the 18 cars in the used dealer change after every race, practice session or online session. They pass through in a FIFO (First in, first out) format.
If i'm bored and want to cycle through the UCD quickly, i'll typically run the Like the Wind Indy race. Takes ~4 minutes to run the race and i'll back out to the dealership every three races to check the new list of cars.
In fact, I might do that tonight. Been looking for one of the old Mercedes 190E's and a couple other cars to try drifting with.