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When DOR walks around at the beginning of the show, you MUST have a MA Sales and Use Tax Registration Certificate visible on your table or you will get hassled. DOR will "sell" you one, but then MA DOR expects to see a business tax return for this year (Schedule C will do) showing all the sales as income and quarterly sales tax returns forever.
This is one reason why private citizens don't rent tables at MA gun shows (like I hear happens at shows in other parts of the US).