If you are a vendor, it doesn't matter if you are doing business year-round or just setting up for a few weeks every year, like a flea market stall or a Halloween store. In both cases, you will need a Certificate of Authority. To cater to both temporary and permanent businesses, New York State issues two types of certificates: permanent and temporary.
You qualify for a temporary certificate if you make sales for no more than two consecutive sales tax quarters in a 12-month period. The New York State Tax Department website lists the sales tax quarters as March 1 through May 31, June 1 through August 31, September 1 through November 30, and December 1 through February 28 (February 29 in a leap year).
In some cases, though, it is better to just get a permanent certificate, since you can only apply for a temporary certificate once every 12 months. Having a permanent certificate will leave the door open for extending your business to year-round operation.
You must not be in business if you don't have a tax certificate. If you are caught, the fines can be up to $10,000.