A broker is one that acts as an agent for others, as in negotiating contracts, purchases, or sales in return for a fee or commission.

【翻译】经纪人是为他人充当代理的人，就像在洽谈合同、购买或销售中做代理，以换取报酬或佣金。

【分析】as in negotiating contracts, purchases, or sales in return for a fee or commission：这是一个省略了主谓语的“方式状语从句”，修饰前面的定语从句。完整的表达应该是：

as he does so in negotiating contracts, purchases, or sales in return for a fee or commission

省略句不等于“无动词分句”。