This building is not in game just for fun. You should be able to buy cars there. Interior of this bulding can be seen in pictures located here as the interior is now used for the Carcyclopedia only.
Question: Can you obtain cars legally outside of missions?
2k answer: No – you receive cars in missions, but you cannot outright buy a car legally. Taking one off the street that you like? Why, sure.
Source:2k forum
As you can see Diamond Motors building was originally marked with a violet neon indicating that there is an option to buy cars inside.