Well, there are situations where retargeting makes a bit of sense, like selling business services on a low-traffic website.

But most e-commerce retargeting, IMHO, is trading on the fact that a naive marketing manager will put too much stock in a high conversion percentage. All of those people were, by definition, aware of the product and actively shopping. Did you create a customer? Probably not, maybe by virtue of them clicking the ad instead of going somewhere else, but that's still pretty zero-sum.

Prospecting for people who haven't previously clicked the item on your site, but would be interested, is a lot harder and potentially more valuable.

All IMHO of course.