[Zak]

I suppose it might be true that the sort of person who buys 50 sets of office chair casters first buys one set to try out, and if they're happy with that, comes back for more in a week. The chance that I might be that person is relatively high reward (a ~$1000 sale), and Amazon doesn't have a better idea of what I'm going to buy.

[manigandham]

The simple answer is that there's a far greater number of people who saw the product but didn't buy then those who did. Conversions are a tiny percentage that scales with price, so showing ads to everyone who saw it raises that conversion rate.

It doesn't really affect the people who already bought it since the conversion is complete, and the wasted impressions are trivial compared to the cost of implementing more complex tech to keep things in sync.

Perhaps there's a secondary effect of negative reputation from seeing ads for stuff you bought but the data doesn't bear it out.