After 12 years getting your money back seems to be a "victory". Awesome that you still had the receipt! If you wish to pursue the additional value, small claims court appears to be the perfect venue for you.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I agree, congratulations on still having the receipt after 12 years and getting a full refund for your purchase price. I am not sure if there is a claim for compensation of an appreciated value, if the watch had depreciated over that time and the store offered you only the lower amount, you would probably not think that fair. The answer probably turns on what a reasonable person would expect when buying a watch like this from Costco; does one make such a purchase intending that they will own an investment quality item that will go up in value? I don't think there is a clear answer, I agree with my colleague, small claims court may be the way to go.
I have been licensed to practice in the State of Oregon since 1990. I am not offering legal advice regarding your question, only general information regarding the law. You are not my client nor am I your attorney unless we sign a retainer agreement.
You might explore a consumer protection act/unfair business practice claim, which would fit your allegation that they misrepresented the nature of the goods you purchased. I can't speak about the specifics of California law, but there may be a remedy for you there.
Please do not consider this answer legal advice. My answer is for informational purposes only. You should contact your attorney to obtain advice with respect to any particular issue or problem. The answer does not create an attorney-client relationship or privilege.