From what I can see, this would be fine under GDPR anyway, so long as when you buy something in a supermarket they ask you "is it okay if we send information of

From what I can see, this would be fine under GDPR anyway, so long as when you buy something in a supermarket they ask you "is it okay if we send information of what you bought to Google?" Of course it's completely impractical to get cashiers to do this every time someone buys something, so it will likely just be applicable to store reward cards. They're doing this already, and all GDPR will introduce is moving the paragraph in the TOC you sign which says "we might sell your data to third parties" to the top in big letters and make sure you explicitly agree to it (or similar to achieve informed consent). As well as adding some safeguards in place. A point on your example, stores already do sell your personal data to third parties as an additional source of revenue, but instead of giving you a 3% discount they usually just analyse your purchasing habits and throw discounts on other goods which they think you'll be susceptible to buying, so usually instead of you spending less you actually spend more on things you didn't really want before being offered.