But he (the writer) has his reward, for he is trusted and convinces, as those who are at the mercy of their own eloquence do not.

本句，convince 用法有误。它是及物动词，要么接宾语，要么用被动语态。要么用形容词 convincing，意思是“让人信服的”。

根据句意，改为 convincing，作表语。

关于 as 这句，它是比较状语从句。注意 as 的含义：肯定句表示与主句同向比较（意为：正如；也是如此）；否定句表示与主句反向比较（意为：然而；则不然）。

But he (the writer) has his reward, for he is trusted and convincing, as those who are at the mercy of their own eloquence do not.
【翻译】但他（作家）得到了应有的回报，因为他是可信赖的和令人信服的，而那些受制于自己口才的人则无法做到。