If a plumber resells a part for more than he buys it for, then he needs to pay the sales tax over and above any tax he may or may not have paid at the wholesale level.
If this hypothetical plumber wants to calculate the tax on the inventory he hasnt sold yet and send it in ahead of time, that would be perfectly legal as well.
The government never complains about taxes paid early.
If the hypothetical gas station did the same thing, I dont see a problem.
I am not a tax professional.
I have professionals advise me on what is legal.