Ebay collects and pays the tax, not the seller. It's complicated, but Ebay will collect tax anytime the jurisdiction of the seller or the buyer requires it. California always requires collection of sales tax, which means you should be seeing it on every order. The only exception would be if you register as a tax exempt buyer. The rules are here:

https://www.ebay.com/help/buying/paying-items/paying-tax-ebay-purchases?id=4771