I don't know previous owner how do I unlock device

You don't. Its an Antitheft feature. If the preivous owner/seller did not remove the Activation Lock, there's nothing you can do. Return it to the seller if you can for a refund.

You can't.

Return it for a refund and buy one that isn't locked, and make sure you check it isn't locked this time before parting with your money.

You can't if the iPad is running iOS 7.x. These iPad have Activation Lock enabled and require the previous users ID and password. Where did you get the iPad? You need to get a refund as the iPad was probably stolen.

 Cheers, Tom 😉

If you are trying to activate an iPad or iPhone and it is asking for a previous owners Apple ID and password, you have encountered the Activation Lock. This is a security feature that prevents thieves from setting up and using a stolen or lost iPad or iPhone. You have no alternative. No must contact the previous owner to get permission to use the device. If you cannot contact the previous owner return the device to where you bought it and get a refund. You will never be able to activate the device and no one can help you do it.