What do I need to sell my stuff at a California convention?

You need a Seller's Permit to sell anything at a California convention - even if you're selling your stuff in an informal space that is public.

More information can be found at: http://www.boe.ca.gov/info/reg.htm . The webpage indicates that you may register for a Seller's Permit at one of the California State Board of Equalization field offices, and get your permit on the same day. Other options include mailing in an application and filling out a form on the website, but they take more time to complete.

San Francisco Bay Area field offices are in Oakland, San Francisco, San Jose, and Santa Rosa. Other cities of interest include Sacramento and Salinas. The website also lists Out-of-State Field Offices.