You need to own the Office and Vehicle Warehouse to do this missions.
If you want to know when you start import mission if its an collection car you can use this License plates page.
Every car can only exists once in your Vehicle Warehouse except when you have a collection car then it can be that you have 3 of them.
First you need to steal a can and import it in your Vehicle Warehouse.
After it you can do sell export maximum of 4 cars 1 car for each player in one sale.
If you want to do the collection for your career progress then you need to collect the full collection and recommended when you have collected a full collection then sell all the cars form that collection when you have the collection collected it is already counting for your career progress.
If you have sold the cars of the collection and still have 2 of the same cars then I recommended to sell one of them.
Best way to making money is that you only sell the Top range cars and collection cars. If you does not sell the collect. This is because the game only let spawn cars that you do not have in your Vehicle Warehouse except if you have an collection car then can be still spawning a car of the hole collection.