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Unlike desktops and laptops, the iPad has no need for any antivirus or firewalls. Because of how the iOS is structured, everything is sandboxed and unless there are vulnerabilities in the iOS that allows you to do so, you cannot manipulate the iOS without physical access to the device. As for internet security, as long as you're running iOS 7.0.6 or above, you're fine as Apple has patched the SSL vulnerability that allowed hackers to bridge your connection to a webpage. For item purchases, if possible, use an app rather than through the mobile browser. Trade giants like Amazon and eBay have their own apps in which you can buy/sell/etc in a sandboxed environment, meaning no one can access it without physical access to the device.