In some cases it's not even the same person doing the purchase, which adds another layer of complexity. Especially common with services that might be purchased

In some cases it's not even the same person doing the purchase, which adds another layer of complexity. Especially common with services that might be purchased by a business. Lately I'm getting a bunch of seemingly targeted PaaS and IaaS ads. I click on them occasionally. If one turns up a service I find interesting, there's a good chance the sale it leads to will be initiated on a different computer, by a different person. This even happens pretty regularly for consumer stuff. I've suggested travel-related things to my parents, for example, which have led to direct sales: I saw an ad for a sale at $hotel_chain, remembered that my parents are taking a trip in October, tell them about it, and they book. It's pretty hard to track back their purchase to the ad that was shown to me. (Not entirely impossible, though, e.g. giving out different coupon codes is one method that print advertising uses.)