My understanding of the law in Victoria, the Auctioneer can only sign the Contract if he/she has explicit written authority to do so. In other states, the Auctioneer has the right to sign the contract. There is numerous case law confirming this. I don’t believe that explicit written authority is in the standard "auction listing authority". I believe that if the Auctioneer held himself to have that authority, and really did not have it, then he may have committed an actionable offence. I agree with the above comments, if there is no contract, then you walk, or negotiate terms satisfactory to you, for a new contract. It sound like the lease may be a commercial one, hence the reference to going concern. The rent being paid, may include a GST component.
Also, GST is not actually paid by a buyer. The rules state, that a GST registered entity, must send 1/11th of their GST assessable income off to the Tax man on the intervals that are set for them. GST registered entities are allowed to increase their prices by 10% to cover their GST liability, and must issue a Tax Invoice which must clearly show the amount of GST claimed to form part of the charge/cost. This is so that the Buyer clearly knows, and if registered for GST, what they can claim as a GST refund.
I have seen contracts where the Seller includes a provision that if GST is levied at some later stage, that they can then claim that back from a buyer. I would delete that clause.