The two that come to mind are staging and photography/videography. Both require a set of niche skills, and work best when you can become the “go-to guy/gal’ for a set of realtors to hire.
Staging requires an eye for decorating spaces to be very appealing to a wide-variety of potential buyers while not spending much money and yet have the result not look cheap. In reality it means a lot of grubby work... cleaning, painting, patching, moving sofas and end tables in and out of rooms.
Photography/videography likewise requires a lot of preparation and post-shoot editing. It’s a lot more than just pushing the shutter on a “nice” camera with a wide angle lens... you have to make the room look good, then come when the lighting is best and (often) carefully illuminate the room to be most appealing. Biggest hurdle here is everyone thinks he/she is a photographer thee days, but few actually know what they are doing.