You could tell the board and convention chair that as club auctioneer you will skip right over any slabbed merchandise in the auction, and refuse to sell it. Then they could decide now whether they need to find another auctioneer or accept your position. This way you could continue to serve as you always have, the board would not be put on "short notice," and you can maintain your principles. They can put the slabbed chips at the end, if there are any at all, and someone else can sell them if there's anyone left in the room to bid.

Just an alternative you might consider. First and foremost you are my friend and will always be my friend. I will not judge you for standing up for what you believe in.