We used to have a "no silent auction" rule in the Code of Conduct, but it fell victim to pruning when we thinned out and streamlined the CoC a couple years ago.
I think the rule had originally been added to stop a hypothetical abuse of that format, since it's completely up to the seller to honestly sell to the highest bidder. But what if the seller changes his mind/doesn't want to ship to wherever the high bidder lives/just plain doesn't like the high bidder/etc...
But I think the main reason for removing it was because we see very few board auctions anyway, and I'm not aware of their ever being a complaint raised that someone felt they had been treated unfairly in a board auction, so the rule went away.