You may complain to them or sue to recover your money. They can also help you if the seller has gone out of business and is therefore not worth suing. If the goods or services provided do not meet proper standards or requirements you can sue for breach of contract and claim for the return of the price you have paid or compensation of any other losses you have suffered connected to the goods or services. Also if you have been injured because the goods were defective you can claim compensation for your injuries. Your rights in law may be considerably reduced.
Auctioneers can refuse to accept responsibility for the quality of goods on auction.
You cannot back out once a deal is made. You should read all notices carefully and look out for any clauses excluding the auctioneer's liability. Goods bought privately do not have to be free from faults. They do have to live up to their description however, and cars bought privately must be roadworthy. You have certain rights when booking a holiday.