Whenever bidders at an auction are aware of the identity of the other bidders there is a risk that they will form a "ring" and thus manipulate the auction result, a practice known as collusion. By agreeing to bid only against outsiders, never against members of the "ring", competition becomes weaker, which may dramatically affect the final price level. After the end of the official auction an unofficial auction will take place among the "ring" members. The difference in price between the two auctions will then be split among the members.

A ring can also be used to increase the price of an auction lot, in which the owner of the object being auctioned may increase competition by taking part in the bidding him or herself, but drop out of the bidding just before the final bid. In Britain and many other countries Rings and the bidding on one's own object are illegal. This form of a ring was used as a central plot device in an episode of the British television series Lovejoy (series 4, episode 3) in which the price of a watercolour by the (fictional) Jessie Webb is inflated in order that others by the same artist can be sold for more than their purchase price.

In an English auction a dummy bid is a bid made by a dummy bidder acting in collusion with the auctioneer or vendor, designed to deceive genuine bidders into paying more. In a First price auction a dummy bid is an unfavourable bid designed so as not to become the winning bid. (The bidder does not want to win this auction, but he or she wants to make sure to be invited to the next auction).

In Australia a dummy bid (shill, schill) is a criminal offence but a vendor bid or a co-owner bid below the reserve price is permitted, if clearly declared as such by the auctioneer. These are all official legal terms in Australia, but may have other meanings elsewhere. A co-owner is one of two or several owners (who disagree among themselves).

In Sweden and many other countries there are no legal restrictions, but it will severely hurt the reputation of an auction house that knowingly permits any other bids except genuine bids. If the reserve is not reached this should be clearly declared.