It shall be unlawful for any purchase of utilities service to be made without, at the same time, collecting the tax levied in this article in respect to such purchase, unless such seller shall elect to assume and pay such tax without collecting the tax from the purchaser. Any seller who shall fail to collect such tax at the time of collecting the price of any such purchase, where the seller has not elected to assume and pay such tax, shall be liable to the city for the amount of such tax in like manner as if the tax had been actually paid to the seller. If the seller shall elect to assume and pay such tax, he shall pay the same each month to the city in the same manner as if such seller had actually collected the tax from the purchaser.