I spent some years in the Art business. Dealt primarily in Americam prints.
My experience with certificates of authenticity is that they are only as good as the sellers reputation.
It is up to the buyer to proove that there is something wrong with his purchase. Not up to the seller to proove its authenticity.
If in fact there is a dispute you still have to collect. IMHO if you are dealing with someone reputable they will stand behind their item certificate or not.