Selling at a show makes you a seller in that state. New Jersey has no temporary permit: even a one-time trade-show vendor registers at least 15 business days before selling, displays the Certificate of Authority where customers can see it, and files quarterly returns until the registration is ended. Florida makes an exhibitor allowed to sell register as a dealer and collect tax. Other states differ - some issue a temporary permit for the show - so each state's rule is written down once, under Registrations, and every show is checked against it.

A full registration does not end when the show does: returns keep falling due - nil returns included - until it is closed with the state. Close it only when every return due so far is in.