Stick.. I think the tax the dealer mentioned isis the FET tax.
This FET is not assessed to individuals who assemble their own firearm from parts for personal use and not for sale, so you don't have to worry about it. The FET (11-12%) is only due on a complete firearm, and is paid by the manufacturer/assembler or builder (so its included in the retail price of the firearm). Even the lower you bought didn't have an FET as it is a component, not a complete firearm, so your dealer doesn't know what he is talking about.
Other than any sales tax you may run into when purchasing your parts, there is no other tax due when you assemble a firearm for yourself.