Q: iCloud Activation Lock

You need to contact the seller and ask him to remove this iPad from his list of devices. Otherwise, this iPad will be a useless brick.

Find My iPhone Activation Lock: Removing a device from a previous owner’s account - Apple Support

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. You 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.