How many screenwriting books have you read?
Compare that number to how many screenplays you have completed. Is the completed screenplays number 0 when compared to the number of screenwriting books read; let's say you already read more than 5 books?
The point is, just do it. Write!
It's ok for the first draft not to be what you are going to peddle. Just finish it.
All writers know that part of writing is rewriting. At some point, just push that bird out of the nest. Let it go.
If sold, guess what, most likely there will be rewrites; again.