Most phones were sold on over the internet (53%) through online auction site eBay (56%) and classified listings websites (34%), the survey by phone comparison website Good Mobile Phones found.
The biggest category of uncleared data was photos (54%), followed by contact details in phone memory (44%), text messages (37%), videos (26%) and application login information (17%).
Most respondents (43%) who failed to delete data said they had simply forgotten, while 32% said they did not realise that information would be stored.
Mark Owen, managing director of Good Mobile Phones, said the survey was conducted to see how people dispose of old devices and whether or not they ensure that all personal information has been cleared.
He said it was surprising that two-thirds of those who said they had sold on their old mobile phone failed to delete personal data.
This could pose a large threat to personal identity protection, he said, especially as internet-enabled devices can make it extremely easy to find out all kinds of personal details, simply by searching through browser histories.
"Always ensure that all stored information is wiped before selling a mobile on; as at the very least, you don't want anyone flicking through your personal pics", said Owen.
This story was first published by Computer Weekly