Answer: Shill biding
Explanation: In simple words, it refers to a fraud in which the individual manipulates the bid by dealing with two or more accounts handled by his friends or himself etc.
In the given case, Doug is a frequent visitor and is placing low bids from one account and high bids from the other and step back from the high bid after winning to get it at a low price.
Hence from the above we can conclude that the correct option is A.