Greetings prospective vendor at Arisia. Before we can allow you to vend at the convention, you must file for and secure a Massachusetts Sales Tax ID. The law requires you to display publicly that sales tax id on your booth for all convention-goers to see. We strongly recommend you first go to the Internal Revenue Service (IRS) website and apply for an Federal Employer Identification Number (FEIN or EIN), which is a prerequisite to getting a Massachusetts Tax ID number.
The IRS has rolled out a new "simplifed" site to apply for an EIN: https://www.ein-gov.us/
Once you have your EIN, you can register your business to get your MA ID at the Mass Tax Connect site: https://mtc.dor.state.ma.us/mtc/_/
You must bring the final Sales Tax ID certificate from the state to the convention. If you do not have this, by law we are required to refuse you permission to set up your booth or 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.