Legal Question in Business Law in California

This is actually a rather difficult question to answer. It may depend upon whether both the buyer and you are "merchants," i.e., people who make a business of dealing in this kind of collectibles. The applicable law seems to be Commercial Code section 2207, dealing with additional terms in an acceptance or confirmation. I would be inclined to say that the second sentence sufficiently qualifies the otherwise unqualified acceptance expressed in the first sentence to amount to a rejection of the offer and a counteroffer to purchase if and only if you can provide a certificate of authenticity. In addition, I think it would invite dispute to regard the offer as accepted and a contract formed unless and until you clarify with the buyer whether the offer is conditioned upon your delivering a certificate of authenticity or not. Asking for clarification is the safe course.

`looks like a homework question to me...