I have iPad but no Apple id
You cannot do anything technically to get past it.  Your only method is to get back to the seller.  If they have disappeared, then perhaps you might want to entertain the possibility that you have been sold a stolen iPad.  In this instance, then rather than have a very expensive paperweight, you might want to report it to the Police.  There is a slim chance that it has been reported as stolen, and they may be able to reunite it with it's true owner, in which case there is also a chance that they may reward you in some way to make up for your loss, or (perfect scenario) they will have already replaced it on insurance, and will simply unlock it for you and let you keep it.