Dealers are not paying the high reserves still being set by some sellers at auction.
Vehicle auction company Aston Barclay is seeing conversion rates fall.
It says vendors are trying to achieve unrealistic values on their used stock.
Conversion rates of between 35-45% have been typical over the past few weeks as reserves are often a few hundred pounds higher than the market is prepared to pay for ex-fleet stock.
“We are doing more homework than ever to help vendors set realistic reserves on cars and enhance conversion rates.
"Recently we identified a two-year-old Network Q approved Astra sitting on a dealer’s forecourt at the same price a vendor was expecting a trader to pay at auction for an identical car.
"Such examples are not uncommon. The auction reserve was a few hundred pounds too high and the car didn’t sell,” explained Barry Watts, operations director of Aston Barclay Group.
"Low conversion rates cause vendors an additional headache as the number of unsold cars can mount up week after week, making them unable to turn used assets into cash,” he said.