Attempts to benchmark prices fetched at auction with what vehicles are predicted to sell for are almost impossible in a 'topsy-turvy' market.
Andrew Shepherd, senior group auctioneer for Manheim Auctions, warned that disposals were being made in a market 'with no pattern and no real logic'.
The warning comes as Glass's Guide reports trade prices for used cars 'easing back' amid an expected drop in demand for used cars as summer approaches.
Prices are down 2% for three-year-old cars, except convertibles which are at the height of their season.
Adrian Rushmore, Glass's managing editor, said: 'It is important to state that dealers remain in a very positive frame of mind and do not see any major obstacles to business in the next few weeks, even though the volume is expected to decline in the traditionally quieter summer months.'
Manheim Auctions reported that dealers were indeed stocking up with enthusiasm, but with an erratic effect on pricing.
Shepherd said: 'Guides can literally be thrown away at the moment as vehicles are achieving whatever they appear to be worth in each individual auction.
'Offering two identical cars can see a difference of £400 to £500 for no real reason and that is how indifferent the buying market is at present.
'Going into the month of July I can foresee that this market will continue to be difficult. There is a lot of stock around and many other factors will keep the market uncertain over the next few weeks.'
British Car Auctions has campaigned to persuade fleet managers that the best way to understand what is happening to their vehicles at disposal time is to attend auctions.
A spokesman said: 'We are always saying stay close to the actual auction, because that is where the market is changing day by day.'