Greetings, prospective vendor at Arisia! To be allowed to vend at the convention, you must file for and secure a Massachusetts Sales Tax ID. The law requires you to display that Sales Tax ID on your booth for all convention-goers to see.
We strongly recommend that you go to the Internal Revenue Service (IRS) site first (https://www.irs.gov/businesses/small-businesses-self-employed/apply-for-an-employer-identification-number-ein-online) and apply for a Federal Employer Identification Number (FEIN or EIN). That number is a prerequisite to getting a MA Sales Tax ID.
Once you have your EIN, you can register your business to get a MA Sales Tax ID at the Mass Tax Connect site (https://mtc.dor.state.ma.us/).
You must bring the final MA Sales Tax ID certificate to the convention. If you do not have this, by law we are required to refuse you permission to set up your booth, or if you have set up your booth already we must ask you to pack up your things and become a non-vending attendee. No exceptions — it's the law. Thank you for understanding.
If you have a MA Sales Tax ID, but you can't find it, you can get a copy from the Mass Tax Connect site. Login, select the account and then look for "Correspondence" in a menu near the bottom of the page. If you don't see a copy of the letter they sent you with the ST-1 certificate, select "See All" and search for it there.