I'm confused. There is no "Board" of Realtors in California. There is the California Department of Real Estate which licenses and regulates real estate agents. And there is the California Association of Realtors and numerous regional associations which are essentially trade associations. You referred to "my association" a few times so I'm guessing that this was an administrative complaint and your association sided with the buyer.
As a member I imagine you agreed to quite a few things by joining. I don't know what but I'm guessing it would limit your ability to sue. Consult an attorney about that and review your options.
Meantime, a small claims lawsuit against you for money has nothing to do with what the association did. You need to properly respond to that lawsuit. To that end I suggest you study the California courts self help pages about small claims court:
https://www.courts.ca.gov/selfhelp-smallclaims.htm
Did you get served with one of these?
https://www.courts.ca.gov/documents/sc100.pdf