There are many uses of cross-site cookies that do not involve selling users' personal data. The most obvious one is customized ads (as used by Google et al), but shared logins and hosted commenting systems are also common.
> Arguably customized ads is still selling user's personal data
I think this is not true, and that it's an important distinction to make.
Selling a user's data means that a site has taken information the user gave them, and sent that data to a third party in some non-anonymous format. It's an unconscionable breach of trust. When there's some service that tells any site a user visits what that visitor's home address is, that's horrifying. It's like having a friend who forwards your private facebook posts to 4chan.
In contrast, when a service uses personal data to change what ads are shown, the data is never sent to a third party. If you tell Google my address so map search gives local results, then they might use that to filter out ads for stores in a different state, but they won't tell those stores where you live.
I definitely see the difference. However, even in the second case, the user's data is being used to make a profit; the company collecting the data and showing the ads is making the ads more valuable - i.e. making more money off of them - with the user's data.
Again, I totally agree that selling the data to a third party is much worse.