Jon won't mind, just give him heads up.
Just put "F4FF fig Auction" on the title and then proceed to explain what it is on the thread.
What kind of fig and if it is in cuttings or potted or both.
Include some pictures and set up a dealine date and time.
You can choose to have it out in the open for everyone to see or you can have it in private.
But if you do it in private then I would suggest you post the highest bid ( not the bidder ) so others interested can see where they stand. If a higher bid comes up then you post that, so on so forth.
The winner must/should contact Jon about the payment and once Jon gives you the greenlight then you ship the product to the winner.
From my personnel experience I think it is something special you are doing. You will feel you have accomplished something to the fig community.
Good luck