Merchants shall pay an ad valorem tax equal to that which is levied upon real estate on the amount of all goods, wares and merchandise, except for grain and other agricultural crops in an unmanufactured condition, as defined in section 137.010, which are subject to assessment, valuation, and taxation under subsection 3 of section 137.115, which they may have in their possession or under their control, whether owned by them or consigned to them for sale, on January first of each year; provided, that no commission merchant shall be required to pay any tax on any unmanufactured article, the growth or produce of this or any other state, which may have been consigned for sale, and in which he has no ownership or interest other than his commission.