To not make a long post and get straight to the point, when player sets his own vehicle on sale, why not exclude that vehicle from automatic car respawn? At least for the time when owner of vehicle is online.
Basically, player spawns vehicle, sets it on sale and parks it somewhere where people can see it. As long as that player is online, his vehicle doesn't respawn, unless he parks it himself from disc.
Here's a poll for players to state if they would even like this to be a thing: