If you have bought a Money-Boost in the form of vehicles, then you can sell a maximum of 6 vehicles per day (within 24 hours). Otherwise you will get a sales lock from GTA which will last for a few weeks. £
If you can't sell 6 vehicles a day, then you probably already got such a sales block from GTA. This will disappear after a certain time, how long this period is, varies (from 1 - 12 months).
Other search terms:
How many vehicles can I sell per day
How many cars can I sell
How many cars can I sell