The sales tax on the sale price is not assessed. The transfer service is taxable though. Also I believe that Nebraska law is such that when an out of state purchase is made (internet, vacation etc..) the sales tax is supposed to be paid by the buyer when the merchandise arrives in the state.
See also the NE "Use Tax". You probably saw something related to it last year when you filed your State Income Tax forms.
Basically, if you purchase anything where Sales Tax was not collected (internet, mail order comes to mind) .... you owe NE Sales Tax on those items.
Thus, if you bought something off of Gun Broker and did not pay any sales tax anywhere for the item itself, you should be declaring it on the Use Tax form when filing NE Income Tax.
The Use Tax has been around for a long time, but it's generally ignored. And it's damn near impossible for the State gov't to track or prove. Were some news stories about it last year, etc. I generally just do a rough estimate and go with that. Fair is fair.