I feel like this is a little bit outside of HN's normal areas of interest, but it's worth stressing that this seems like it has the potential to be a Big Deal.
That's for a few reasons:
1. The display advertising ecosystem has, for years, been talking about data-driven display. The truth of the matter is, though, that most of the data is indirectly inferred and extremely low-quality. Companies like Google and Facebook tread very lightly when it comes to offering data for targeting for fear of massive privacy backlash, but it seems like Amazon could sidestep that. If Amazon is allowing implicit/indirect access to Amazon purchase history, it will be far and away better than anything else out there. More importantly, the unevenness of the data and of performance has really held back broader adoption -- if they're smart they may be able to bring over many of the holdouts.
2. Advertisers and agencies have an extremely wary relationship with Google and are cautious about Facebook too. If they play it right, Amazon can be the major non-aligned player in the industry -- a technology without inventory.
It's also worth noting that I've recently noticed Amazon ramping up its own retargeting programs more aggressively. Assuming that retargeting performs for them like it does for virtually every other e-commerce site out there, I would not be surprised to see a significant jump in Amazon's revenue attributed to that over the next few quarters.
There should be some very interesting opportunities for startups to piggyback themselves on this when it comes out.