Online auction site TradeMe is promising to crackdown on shill bidding - a term that describes artificially inflating the price of an item.
Sellers often use friends, family or employees to bid for an item and boost its price.
The auction site was involved in two high-profile shill bidding cases last year, when the Commerce Commission prosecuted two motor vehicle dealerships under the Fair Trading Act.
TradeMe head of trust and safety John Duffy says the dealerships listed vehicles with a $1 reserve, and then shill bidders drove up the prices.
He says the cases combined led to buyers paying $140,000 more than they should have.