You need to own the Office and Cargo Warehouse to do this missions. I recommended to buy the Large Cargo Warehouse because when you have a higher amount of filled your Cargo Warehouse then you will get more payed per crate. When you have the large filled with 111 crates then you will get $20.000 per crate.
First you need to steal cargo and paying for it or let your Staff Member source cargo for you. on the map there is an human icon. This takes 1 GTA day (48 minutes). Sometimes you can source special cargo you will get more money when selling for these.
The Staff Member will sometimes source special cargo.
After you have done this and you found now its the right moment to sell it you can start a sell mission then i recommended to sell all crates at once. for the sell mission you can get planes boats or cars. There will always drop multiple vehicles.