Wether it's legal or not, I wouldn't know. Wether you are ABLE to resell it, that's a different matter.
When you sell your ipad with apps in it, your account must be connected to the ipad and itunes. If the one you sold the iPad to would want to sync with their itunes, all your apps will be removed unless his computer is authorized to use your account. And since you can only authorize 5 machines (That's iDevices and computer) you'd not be able to sell your apps more than 3 times..
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