Can an agent sue a buyer?

Lawsuits can be brought by anyone for anything, so yes, a real estate agent may sue a buyer.

Now, there has to be a good reason to do so in order for the legal action to not be considered "frivolous" and thrown out early in the court process.

An agent may consider bringing a civil lawsuit against a buyer if the agent believes that buyer misrepresented themselves during the purchase process (resulting in an incomplete transaction and a loss of commission). The transaction, however, would need to be large in order to warrant such costly action.