9. To furnish requested advice to the best of my ability and knowledge, and not to take advantage of superior knowledge on my part to the disadvantage of a less knowledgeable collector.

If the seller didn't ask for advise and you didn't tell someone "it's worth $x" way under what you knew it to be worth, to take advantage of them, there's no problem.

I could be wrong but I believe the intent is that someone who knows a chip is scarce shouldn't tell a seller, WHO ASKS, that it's common and cheap.

Honest advise is the key.

If I put up a chip on eBay for $5 BIN and it's worth $25, that's my choice. Maybe it's only worth $5 to me.

If a seller wants to check ahead, they can ask and should expect honest answers. In your case, the seller just placed the auction, of his own free will.