Any local business, service or place with a physical or virtual presence is fair game. So, even though the guy you buy used textbooks from on eBay or Amazon is amazing and has a 99% response rate, he doesn’t qualify as an interaction medium.
Think: restaurants, banks, grocery stores, bars, doctors, hair salons, etc.
Be sure to remember your No. 1 priority as a reviewer: Focus on deaf-friendliness (or lack thereof) first, and other details second.