Regarding selling something for someone else:
A person can sell or auction whatever they want under their own name.
Where they got that item and what they do with the money they receive for it is their own business.
So, if John has permission to post auctions and Peter does not, Peter can ask John to auction his Charizard * for him
However, it is John's auction. The Charizard * is being auctioned by John. It is John's responsibility to get the Charizard * to whoever the winner is.
There is no "well, I was selling it for Peter, so I don't know when he's sending the card", It's not the buyer's concern where the card came from or who John is passing money on to.
So, John will post:
"I have a Charizard* for auction, bidding starts at $5.00 (or whatever)" and that's it.
John should not be posting "I'm selling this for Peter".
Not our concern. John's rep is being depended on and is the rep that is on the line.