Wha?... I don't understand.
It would help if you would finish your sentences before you would begin another.
From what I can see, these seem like the mentioned scenarios:
When Person A buys a car, and Person B tries to steal it, you want it so that Person A cannot respawn it.
When Person A buys a car, and Person B buys it from Person A, you want it so that Person A cannot respawn it.
This idea seems flawed, unless you make it so that it checks for money and does not allow players to build over a certain limit. Vehicles would have to clear when the owner leaves, but it would have to either give the money back to the player, or make the vehicle and it's position save, so that when the owner rejoins and spawns, they get the vehicle back, right where they left it. Would also need support for Admin codes, bla bla bla...
If not you have people spamming vehicles everywhere.