If vacuum sellers are willing to bid top dollar to advertise to people that just ordered a vac, who is Amazon to say they are wrong? Maybe they are betting on a dissatisfied return by people that clearly were in the market for a vacuum. Maybe the transaction isn't data that is available to the bidders.

Advertizing works. even on those thar believe themselves immune.

Far more smart systems already exist and are already used to sway people on a large scale.

The smartest system might not be the most profitsble for the intermediate add platform owner, so in those cases you might be on the receiving end of 'stupid' adds because it means more profit for the platform owner.