You may not sell any content that gives the customer an advantage over other players.

This of course includes (faster) vehicles. However the exact line where this starts I do not know. This area is kinda vague in wording in the ToS.

E.g. when you can buy those same vehicles ingame for a reasonable amount of money compared to playtime, this should be fine.
(But as the guy above me already said: this is just my thought process here and might not be 100% accurate)