Slastdot has a post about an old computer that had been used for running POS software in a retail business which was sold on eBay. Here’s what was found on that old computer: default passwords, a security flaw, and names, addresses, and social security numbers of employees of the terminal’s previous owner.
The safest way to get rid of a computer you no longer need is to take a hammer to the hard drive or solid state storage. Smash it to bits. If you do sell it or give it away, format the drive first.