Informit did a test in 2004 to see how much data was still available on hard drives sold on eBay. The results were shocking, many of these drives still contained confidential data like social security numbers, bank account information, medical records, ... All of this was fairly easy to recover, even if the drive was formatted.
Now in 2007 the site repeats the experiment with memory cards:
Our budget for this project was roughly $100. While limiting, small capacity cards are relatively cheap. In all, we spend $70.47 on a selection of 16 cards (plus another $42.60 on shipping!). Of these, one got lost in the mail and another was dead upon arrival.
So what did they find:
Statistically, this indicates that 78% of the cards we obtained on eBay contained recoverable data. In total, we found 240 pictures, 17 movies, and a wide range of files from the card with computer files. The evidence suggests that people are not aware that their privacy is at risk. In addition, the fact that some of the cards contained undeleted images is a bit disconcerting. At a bare minimum media card owners should have deleted the viewable images.