New figures from Manheim show dealers achieved an average £2,629 for part-exchange stock at auction during August.
Despite a rise of one month in average age, and 1,327 in average mileage, the average selling price in August fell by just 2.8% (£74).
Compared to August 2012, there is strong demand for part-ex vehicles, with prices up 6.5%.
Looking at the figures in more detail, the executive sector saw a rise of £19, despite an average mileage increase of 863, to 100,846.
The biggest value decrease was in the coupe sector, where selling prices fell by £333 to £4,602, again with higher age and mileage.
Daren Wiseman, valuation services manager at Manheim, commented: “The fact that part exchange values remain fairly strong indicates a continued shortage of quality cars coming to auction.
“With the September plate change, which typically accounts for around 17% of the full-year registrations, it is likely to mean significant numbers of new vehicles feeding the starved market.”