Dealers have been criticised for an overreliance on CAP values, which Motorpoint suggests are "10% adrift of the real market valuation on many models".
The car supermarket seller said the current "obsession" with achieving CAP clean means the trade is buying stock from the auctions for more than it retails the same cars on its sites.
However, CAP points out it has already adjusted prices in line with the market and is happy with its current guide prices.
“We are confident that Black Book for June was reflective of typical values across the main relevant conditions of Clean and Average based on the mass of information gathered during May,” said a spokesman.
“In a weakening or indeed rising market a monthly price guide will inevitably be overtaken by further market movements over the course of a month and we are confident that CAP users understand and take this into account once a trend has been clearly established. It remains a core principle of CAP that it reflects rather than leads the market.”
Motorpoint said it is selling an 09-Reg Chevrolet Aveo 1.2 LS with 11,000 miles for £4,999, but points out the same car CAP ‘cleans’ at £5,100 and CAP suggests dealers retail it for £6,295 - nearly £1,300 more than Motorpoint.
It also points to a Vauxhall Astra 1.8 SRi , which it is selling a 59-Reg model with 12,000 miles for at £9,999.
The same car CAP ‘cleans’ at £10,300 and CAP suggests it retails for £12,250 – a difference of £2,251.
But CAP said: “In terms of the impact of CAP values on retail customers we doubt that Black Book suggested retail prices cause dealers to offer poor value to their retail customers. The vast majority of used car retail dealers will determine their own forecourt prices, based on wholesale purchase price, relevant prep costs and their own business model.
Indeed, this is what Motorpoint does, as David Shelton, its managing director explained: "Motorpoint has never had the same problem as traditional car dealers, preferring to rely on its own pricing data rather than CAP when buying its own stock.”