Connect shuffle. Run iTunes. In the sidebar, select shuffle under DEVICES. Over to the right, go to the Summary or Settings tab (depends on shuffle model). There should be a Restore button. Click it to erase the shuffle, re-install its software, and set it to default settings.
When it start up and connects to iTunes, decline to do anything that syncs music to the shuffle. The new owner should be invited by iTunes on their computer to sync the shuffle to their computer's iTunes library, the first time it is connected.