This can be done 'quick and easy' in "Sandbox" mode (unlimited money, all skills available, and no customer cars) if you wish to obtain this trophy outside of your story playthrough.
You can still follow this guide if you wish to do this within your story/normal mode.
To get to Sandbox:
--> main menu
--> profiles
--> create new in an empty slot
--> select the new profile
--> choose "Sandbox" mode
Buy cars from the auctions, salon, barns and junkyard, then sell them once you get back to your garage. You can buy many at once (like i did, after you upgrade parking just because that helps!), then just spend the time selling them all, or sell the cars everytime you get back to the garage.
To sell a car:
--> have the car in the garage, it can't be in parking
-->
when next to the car
--> toggle
to "Car Status" (green icon)
--> press
on "Car Status"
--> press
again to sell, and then to confirm
When in Sandbox mode, you don't exactly have to worry about running out of moolah, so you can buy all the cars you want
This can also get you the Dealer Silver and Gold trophies, if you can stand doing the same thing over and over for a nice while
