It seems that apple have removed https://www.icloud.com/activationlock which you used to be able to use to check the activation lock status of used phones before purchasing them. Maybe apple removed it to stop assisting the used phone market and force people to only buy brand new phones instead. Either that or it is only down temporarily. Otherwise if you are unlucky enough to get a locked phone, you can unlock it manually by changing the serial number - but that is quite hard to do and if you are unskilled you can destroy the phone in the process. So hopefully the website will come back because it was useful.