When buying a grey market watch there is always a question of authenticity and there is usually no official warranty from the manufacturer because their policy states that the warranty is only valid if the warranty certificate is dated fully and correctly completed and stamped by an official dealer

What is the correctly punctuated version of this sentence?
When buying a 'grey market' watch there is always a question of authenticity and there is usually no official warranty from the manufacturer, because their policy states that the warranty is only valid, if the warranty certificate is dated, fully and correctly completed and stamped by an official dealer.