Don´t know, why PD made the decision to cut the feature of selling most of cars you get. In previous titles, you couln´t sell concept cars, but everything else.
Now (and that is worth a laugh), you cannot even sell race cars, you bought yourself.
In most cases, I sell or remove unwanted cars, even when I don´t get money for it. I only want my most wanted cars in garage, standard cars almost have no chance. Of course, you can fill your garage wih every car and link your favorites with the "favorite"-button.
But in my case, picking my all-time-favorites out of a huge selection is the sense of Gran Turismo, not collecting like a messy