The Calif Board of Equalization(fancy title) requires all vendors to collect and remit taxes. There are no exceptions if you are there in person, not on the internet. The rate varies by county but will run about 8-81/2%. If BOE comes to the show, as they do to Knife Expo, and you do not have a permit, they can either issue one or close you, their call.
When I do Calif knife shows, I put a card on my table that says state sales tax will beadded to all sales. To do otherwise and eat the tax means you are giving a 10% discount. You can mark the item up and do it that way, but Calif is really rabid about collecting thier money.
All you need is a temporary sellers permit. It costs nothing and reduces large risk. Go online to the CA BOE website, it's all there. They can issue a permit and fax you a copy within a few days.
You can go without and take a chhance, but with my luck it would be the first time they did a raid on the show and I'd get the boot.
We all hate the taxman, but we still have to live with him. And personally, I won't discount my knives 10% for him.
Pardon my typing. My disk grinder and I had an arguement and it's hard to type with a bandaged finger. The tangs are all flat, though.
Gene