Carcraft, one of the UK's biggest used car supermarkets, has been investigated by the Office of Fair Trading (OFT).
The action followed concerns that its business practices, finance and after-sale guarantees weren't as advertised.
Cavendish Elithorn, senior director of the OFT's Goods and Consumer Group, said: 'Carcraft promised customers that it aimed to help them "find the perfect car with confidence".
'We don't think its practices ensured that it always lived up to expectations or met its legal obligations.'
The investigation looked into claims that Carcraft wasn't properly carrying out comprehensive pre-sale vehicle inspections and that it didn't always meet legal obligations to repair or replace unsatisfactory cars.
There were also concerns over customer confusion regarding finance payments and misleading after-sale vehicle guarantees that customers believed were free of charge, but ultimately weren't.
Carcraft co-operated fully with the OFT's investigations and has confirmed it will address all the issues raised.
A Carcraft spokesman said: 'We take customer service and satisfaction extremely seriously and we are genuinely sorry that in the past some customers didn't receive the levels of service we currently offer.
'We have worked with the OFT to produce a set of comprehensive voluntary guidelines that will set a new standard for the industry.'