Every retailer wants a retail media revenue line — most are building the wrong layer first. The ad-serving technology is commodity; what makes retail media commercially credible is the supplier data infrastructure underneath it: real-time stock and pricing visibility, transparent campaign performance, and trade promotion ROI suppliers can actually see. Drawing on infrastructure built at one of Europe’s largest electronics retailers — processing millions of price points daily and generating measurable supplier revenue at scale — we show what the data layer under a real retail media programme looks like, and why most retailers are chasing the wrong part of the stack.