Read the two threads linked above and ask any further questions there.
A car stored in your garage is a personal vehicle.
When you over sell personal vehicles you will be punished by the daily sales limit.
Rockstar implemented it into the game quite a few years ago.
Depending on how many cars the game will allow you to sell in a 30 hour timeframe determines wether or not this can be reversed.
At this point you should stop selling cars immediately.
Read the OP of those threads and fully understand what it is you have gotten yourself into.