About all I know about finding a good agent is that you should always ask for and follow up on references, and it would be probably help to ask the agent specifically if they have any references who can speak to their ability to work with big, expensive, serious house sort of properties (bonus points if they've dealt with a similar situation to your parents'). Also it's important to find an agent who's reasonably web-savvy, since most buyers are looking for listings online.

Best of luck to your parents!