Correct, this is how some data aggregators sell your purchase data without actually selling your receipts. You will have a number of marketing id's from different aggregators, but people like Amazon, eBay etc will aggregate from multiple sources and can track you across aggregators. And there are re-targeting and re-marketing companies that do this as a service.

It lets them say they never sell your data, but they do sell meta-data about you. Probably a little over simplified but you get the idea.