I'm pretty sure one would have to obtain a tax ID number in order to collect tax.
I don't have one, and I don't charge tax when I have a table at a gun show.
I am however, only selling personal items. It is not my "business".
YMMV
Nebraska sales tax laws do allow an exemption for "occasional sales". However it looks like selling at a gunshow would not meet the criteria of the law.
REG-1-022 OCCASIONAL SALES
022.06 Occasional sale includes a sale of household goods and personal effects by an individual when such property was originally acquired and used for personal use.
All such sales must occur at an individual's residence and may include the property of more than one person.
022.06A Such sales may not be held on more than three days during a calendar year at any residence.
022.06B Such sales may not include property of a type which is similar to that sold by the individual or any member of his household in a trade or business.
http://www.revenue.ne.gov/legal/regs/slstaxregs.html#022So it looks like it's a matter of if the "tax lady" walks around and discovers you. Not sure how they can make you collect sales tax if you don't have a sales tax ID number though...