Many people sell their computers without being aware that they still contain some very sensative personal information.
A lot of people are under the false impression that all information is deleted when a hard drive is reformatted.
Security experts suggest that you should not sell a computer if you can't securely delete your data. There is software that will securely remove data and there are professionals who can do it for you.
This article has more details
Second-hand PCs still full of sensitive data - vnunet.com