What?
You go to bspec, if you haven't created a driver it will ask you to create one. You go to a race, change to a car in your garage that is allowed in this race (if you haven't) already, you then enter the race, pick your driver, and go.
Nothing to do with selling cars or needing different cars. It just sounds like you aren't picking your driver or something... before each race it will show you your driver(s) and you nave to select one.