I collect the sales tax from all my customers. They don't have to register. I do. I pay that sales tax that I collect directly from my checking account.
The sales tax is always paid by the final user, never a merchant. The merchant, however, in order to be able to sell, is obligated to collect the sales tax from the sales tax payer and pass it to the government.
The JWs position in the sales chain would be the same as you? And you are registered?
Is there a minimum amount required before you need to register as a 'merchant'? It would strike me that the individual JWs would each be below such a threshold due to the low-valve of sales anyway.