how do I report a sitter taking payments outside Rover?

Since Rover Support sees all of the client/sitter communications, that's all you'd need the first name of the client and sitter. Since this topic is posted on the Q&A forum now, they may even contact you to request the details.

Adding to Deb's answer, you may also need your Meet and Greet's name; it would then be easy for Rover to match sitter and client to narrow the possibilities.