He might have a good case. The sales tax law in many States specifically says that sellers have to collect the sales tax from consumers, which the State then collects from the sellers. The law cannot state that the State can collect the tax from “whoever” they can tax in a transaction.
And if the court rules that the State can collect the tax from consumers, then sellers are no longer under any obligation to collect the tax themselves, as long as they file a record of the taxation for the sale with the State. Let the State then send out bills to all consumers for every transaction that they make.
But the State cannot have it both ways.
They can and they do. The states have two laws when it come to taxing transactions. They tax the seller and when the seller is not taxable they levy the tax on the user. This is old established law and it is not really up for discussion.