Taken from the Support forums, credit goes to "Suizide42"
How To: 1. Have your friend sit in a public or private lobby. Your friend needs to stay in this ONE lobby for this to work 2. Sell your car and immediately click Start > Online > Find New Session 3. Either be invited, or join your friends session 4. Car is still in your garage in the lobby 5. Jump to step 2 again 6. Rinse, repeat, profit!
The only thing to be mentioned about this is that this only works as long as someone is in one "persistent" lobby. By this I mean, as long as someone is in a lobby without leaving it, that lobby seems to not recognize the car being sold. The second that lobby is closed\your friend leaves, you will no longer have your car after that latest patch.