“If you found this product, there would be no guarantee that it would be genuine.” First "would": The main result of the "if" condition ("there would be no guarantee"). Second "would": Describes the nature of the guarantee itself. The guarantee, if it existed, would state: "This product is genuine." But since the whole situation is hypothetical, that statement inside the guarantee also becomes hypothetical: "it would be genuine." The second sentence you've provided is a single, complete past hypothetical. "Would have depended" is the standard form for an unreal past (third conditional) and covers the entire idea.