Called the Manheim Challenge, it encourages used car managers to visit Manheim Bristol, Gloucester or Leeds on specific days during November and guess the value of eight individual cars going through a sale.
A Manheim dealer development manager will also value each car and the results will be compared once the auction has finished.
A total of £3,000 in prize money is available and although Manheim bosses say the exercise offers a bit of fun, there is also a serious message.
Manheim dealer director Alan Cureton said: 'Too many used car managers rely on their guide books or their local car trader to value trade-ins and we believe they are potentially undervaluing many of their cars to the tune of tens of thousands of pounds each year.
'In a market such as the one we have experienced this year where demand has exceeded supply, market values on many cars have been beating the used value guides by 10% to 15%. Unless you are visiting auctions regularly you will undervalue a lot of used stock.'