i need to factory reset my imac late 2009 i have sold it can someone advise how to pls

My info shows 10.6.1. Yes, you will either need the original discs or a retail copy of Snow Leopard. However, the pre-installed bundled software is only on the original discs, so you should provide those to the buyer. You can purchase replacements from Apple Support:

Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.

To contact product and tech support: Apple - Support - Contact Apple Support.

Mac OS X 10.6.1. You'll need either the computer's original disks or a retail Mac OS X 10.6.3 DVD, which is available from the online Apple Store.

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Throw away any files which may contain sensitive data, securely empty the Trash, insert its original disk, restart the computer with the C key held down, use the Disk Utility to erase the partition with Mac OS X installed, install a fresh OS onto it, and include the disk with it.

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You won't be able to use that combination of disks on a late 2009 iMac. If you don't have its original disks, phone AppleCare and order replacements or buy a 10.6.3 DVD; the retail DVD won't install iLife.

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What to do before selling or giving away your Mac

You cannot use either of those. The Leopard disc cannot boot the computer. The Snow Leopard upgrade won't work unless Leopard is already installed.