Citizens Advice Scotland (CAS) is being contacted by almost 100 Scots per week who claim to have been ripped off by car dealers.
The organisation says one in 10 second-hand cars are unsafe to drive and 69% develop a problem in the first three months of ownership.
It has said staff dealt with 4774 complaints about used models between October 2012 and September 2013.
CAS's analysis of 190 calls in September found the total value of the cars affected was £1.2 million. Citizens Advice Bureaux is launching a UK-wide campaign to highlight the issue.
Chief executive Margaret Lynch said: "What we are finding is that used cars are one of the biggest areas where consumers are being exploited. At a time when many Scots are struggling financially and looking for ways to save money, rogue car dealers are cynically ripping them off by selling them vehicles they know are duds."
Scots-born British Touring Car champion John Cleland, who is backing the campaign, added: "There are a number of quick and easy steps people can take to make sure they are buying a decent motor."