It depends on how certain you need to be.
You could just delete the files and clear your browser cache and history it's not that difficult for someone technical to recover that or parts of that if they have the time and inclination
As Scotsman says you can format the hard disk (not a quick format)- this will delete every thing - things can still be recovered with special kit.
The erasing tools write patterns repeatedly over the disk - in theory making it totally irrecoverable - with a Government size budget? maybe something could be recoverred.
I work for a security company - our hard disk are distroyed physically in crushers.
You pay's your money and takes your choice
If the buyer is expecting the operating system to still be on the machine though they may not be too pleased if the hard disk is formatted and they need to buy and install a new one