1 item can be for sale and advertised for sale in different ways, and that is legal. It's exactly what cash converters does, as do many other stores.
They are allowed to auction the tank and have it for sale in store; that changes when the auction reaches reserve and the bid-offer contract is sealed. liability of proof then needs to show that the sale was made before the auction reached reserve.
The only thing wrong with what has happened in this case is that they did not specify that the tank was advertised for sale by other means, but being a known retail store it would be assumed that the tank was not only for online sale as it did not specify online special only.