Q: How do I activate a previously used iPhone

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

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

If it is still asking for activation obviously device was not removed from the list or you have entered incorrect info. I would recommend to contact the previous owner. It is the only possible solution.

Tomessi wrote:

 

I have and the device was removed from the owner's account but the phone still asks for activation.

It was not. If the previous owner does not remove it, then you will not be able to activate that device. Apple will not help you circumvent Activation Lock.