I think that's the exact problem to which GP poster is referring: tracking only knows about URLs visited but not purchase history. A visit to the product page "vacuum cleaner WheezyMatic 3K" will get you flagged as interested in that product and you'll get shown ads for it — even though you already bought it on the original site.
Maybe the ads could have content along the lines of "The WheezyMatic 3K: The Best On The Planet" and so when someone who already has bought the product sees those ads, they're instead reassured about their purchasing decision.
