You are buying direct, rather than through an agent.

你这是直接购买，而不是通过代理人。

...a written declaration by someone, authorizing another person to act as his agent.

一份某人授权他人为其代理人的书面申明

All these years he's been an agent for the East. [+ for]

这些年来他一直是为东方国家效力的一位间谍。

In many societies young men regard themselves as highly active, the agents of change, shapers of the world.

在许多社会中，年轻人将自己看成是高度活跃的分子，是变革的推动者和世界的塑造者。

...the bleaching agent in white flour.

白面粉中的漂白剂

...fibrinogen, a blood clotting agent.

血纤维蛋白原，一种凝血剂