One of the reasons I have come to LA almost a week before the Unique LA gift show is to obtain my 'temporary seller's permit' from the Board of Equalization. In California everyone that sells something (that includes people that have more than 2 garage sales a year) needs to file for a seller's permit and then pay the state taxes after the event is completed. As a Canadian I was worried that I wouldn't legally be allowed to sell in the US but it turns out, when you are aiding their economy, the American government makes things easy. Today I drove across LA (scary) to Culver City and got my permit with zero problems. I owe most of this ease to a very organized exposition administrator for an Anime trade show that was held last summer in downtown LA. Whomever was running the event actually goes step by step through the long and often misleading application form guiding you the whole way. Awesome.
If you are in need of a permit go here for all the info necessary.