"如果这个明星的可信度很高的话，比如成龙，我想我会相信他代言的产品。"怎么翻译？

If a star is of high credibility,such as Jackie Chan,I think I will buy the product which he represents.

难说

哇、英语高手才能译。

If this star, then the credibility of a very high, such as Jackie Chan, I think I would have believed him speak of the product

"如果这个明星的可信度很高的话，比如成龙，我想我会相信他代言的产品。"
If the celebrity(star) got a huge amount of credibility, such as Jaki Chan, I think I would buy it since he is the spokesperson of the product.

注：
在这里用"I buy it" 是“我赞成；我相信”的意思。（习惯口语用法）
"since" 这里表示：“既然；因为”的意思。