A seller may collect the sales tax from the customer if the parties agree to it as part of the sale. In your case, I’m sure the dealer would prefer that you foot the bill for the sales tax they will owe if they sell the vehicle to you in California.
If the dealer delivered the vehicle to you outside of California - as opposed to you taking possession within the state - then they would be eligible for an exemption from the general rule.
See the link below for some highlights of the rules, and good luck in your negotiations.