I think what you're referring to is different or more advanced. Simple remarketing involves putting a tracking pixel on your website, and then your ads show only to those users on another platform such as Facebook. It usually (on the base level) does not include purchase history.
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.
