Why would people want an ad for something they bought already? Hardly ever unless it is only about weekly shopping. And then they don't need to be enticed to buy it as it is already on their shopping list, evidently. For one-time-only buys it is just as useless for the opposite reason.
In my experience, that makes it more annoying - yes targeted, but too late/useless. To make it useful, they'd need to predict buys. And that can be pretty creepy, like with the pregnancy stuff. If they get it right, which is really hard.
The Amazon 'other people that looked at/bought this also bought Y' is occasionally useful, when the items deal with a specific well defined topic (say learning about a certain programming language), but even there it has a lot of misses. For fiction it can be really off-putting even.
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