Agent Bought The House

If the agent was not the managing broker in that company, then her supervising broker is the first person to talk with about the matter.

Then you can contact the local Association of Realtors. That’s a private organization concerned with the ethical practice of its members. I’m assuming this agent belongs, most local brokers do.

To make more trouble, your folks can take a complaint to their state’s real estate commission, which issues licenses.

To be fair, if the house was multiple-listed, many brokers had a chance to market it to their buyers, and evidently they didn't. Add to that the fact that your folks “had trouble selling it,” and we must suspect that their asking price was too high, much more than market value. So perhaps they weren’t cheated on the price.
Some experts on the Law of Agency, though, believe that the agent who buys property and also collects a commission on the sale is practicing illegal dual agency, with a conflict of interest.

You may want to consult a lawyer who specializes in real estate, but remember that an attorney's first question is often "What are you trying to accomplish?" Do you simply want to make trouble for the agent? Try to have her license taken away? Reclaim the amount of the commission? Bring in an appraiser for market value and sue for any difference in price? Cancel the sale and take the house back?