thank you. If your bid and subsequent payment in relation to your winning bid have been accepted and confirmed by the auction house-and it is important to stress the importance of your order been confirmed. This would usually be by email for example. if you have not received confirmation of acceptance of your order, then the contract may not be final and in the circumstances it may be open to the auction house to propose additional or alternative terms.
if this is the case, then as you say, your order would be confirmed on confirmation of your order and from that point, cannot be unilaterally changed by the auction house. this is the fundamental basis of a contract - if a contract can be changed at will by one party having agreed it, there would be almost no purpose and contract law.
If they are refusing to deliver the goods at the price agreed and confirmed under a confirmed order, then this places the auction house in breach of contract. if this is the case, you could either demand a full refund in lieu of paying anything different or in addition, you could seek to claim any difference in price as between what you have agreed to pay for the goods from the supplier and any alternative supplier as damages for breach of contract.
if you require a refund, if you paid by credit or debit card, the simplest way to obtain a refund is likely to be to contact your card issuer in this respect to ask them to process a chargeback if you have requested a refund and it is not been supplied by the auction house