This paper uncovers issues in the design of camera-based technologies to support retail shopping in a physical store, specifically clothes shopping. An emerging class of technology is targeting the enhancement of retail shopping, including the trying on of clothing. Designing such systems requires careful considerations of physical and electronic design, as well as concerns about user privacy. We explore the entire design cycle using a technology concept called the Responsive Mirror through its conception, prototyping and evaluation. The Responsive Mirror is an implicitly controlled video technology for clothes fitting rooms that allows a shopper to directly compare a currently worn garment with images from the previously worn garment. The orientation of images from past trials is matched to the shopper's pose as he moves. To explore the tension between privacy and publicity, the system also allows comparison to clothes that other people in the shoppers' social network are wearing. A user study elicited a number of design tradeoffs regarding privacy, adoption, benefits to shoppers and merchants and user behaviors in fitting rooms.