1. If it is indeed a part of common area then the builder can not sell it to any fat owner including you.
2. SO if he has sold from this area to various flat owners then he can sell to you as well.
3. Now it is to be seen whether he is selling an extra parking space at the cost of allocation of other flat owners.
4. if not then he can sell another parking space form the excess area remaining vacant.
5. the open slit parking space is considered to be a common area not the covered parking space.