One of the other things to consider* is that the details of person X that is looking to buy item Y are a snaphot at the time that information was brought/sold.

When person X buys item Y from company Z, company Z is unlikely to broadcast that fact. Ergo, the 'system' doesn't know that you have now brought item Y and that you no longer have an active interest in purchasing Y.

*Things may have changed in 20 years so the above is just an outdated observation based on a brief stint at a turn of the century B2B marketing start-up.