The really crazy thing is that tracking from click to conversion is still not a truly solved problem!
e.g. When a person clicks, and then closes the browser and/or views other web sites, only to come back as direct traffic later and convert. The funnel is very hard to track and very easy to lose.
Not that I do this full time (I don't), but I've yet to really see a solution that lets me truly match up a person who clicks an ad and then converts.
At best, I can roughly correlate ads to a bump in revenue.
Your example is actually pretty easy to track. That's why sites drop cookies and have look back logs.
Harder to track is when someone clicks on a link on mobile, and then switches to desktop to complete the transaction. Unless everyone is logged in, you have no chance of tracking the funnel.
I know nothing about advertising, but isn't this method thwarted by users (likely small in number) that either do not allow cookies or dump cookies after a session? The latter type would only affect tracking if the transaction was completed in another session. I would think that IP tracking is somewhat useless due to shared public address among users.
Really what I'm asking is that is it trivially possible to track a user that does not store cookies?
Yes, dumping cookies makes it much harder to track lookback conversions. Some networks will use fingerprinting (IP, user-agent-- panopticlick style stuff) to supplement their data, but it's much less reliable and a determined user, like someone blocking or dumping cookies, is going to defeat it.
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.)
e.g. When a person clicks, and then closes the browser and/or views other web sites, only to come back as direct traffic later and convert. The funnel is very hard to track and very easy to lose.
Not that I do this full time (I don't), but I've yet to really see a solution that lets me truly match up a person who clicks an ad and then converts.
At best, I can roughly correlate ads to a bump in revenue.