If I overpaid a travel agency for a ticket, can I claim some money back? How?

I'm not sure how you intend to prove that the agent bought you the same ticket (the basis of your claim); and pocketed the difference.

Consider the following:

It is normal for airlines to offer cheaper fares on their website than agents; or hold different promotions that are not offered to agents.
The fare type/class/code may be different; you may not be comparing apples to apples; even if both tickets are for the same class of travel.
Rates that are available to individuals through direct booking are not always available to agents.
The agent would have to have advance knowledge of a booking; hold your reservation till that date, ensure that they were able to book that reservation in order to take advantage. A highly unlikely scenario.

I personally have had to pay a premium for booking through agents many times.

Burhan Khalid 39437

Source: stackoverflow.com