To clarify... As far as I know the only cars you can't buy used are the one sold in the new dealership. Aside from that the cars in the online dealership are basically some of the more commonly sought after cars from the regular used dealership.
Yeah I've seen quite a few of the prize cars show up in the used dealer but typically they are $$$$$$ compared to what you sell them for. So I only tend to sell them if I really have no interest whatsoever in the car.