"When others explained that Facebook and Google didn't sell customer data, they didn't believe it"

I'm not sure there's a significant meaningful difference between direct selling and what they actually do, which is to make it available to target and manipulate people with extreme granularity. This is a huge part of why a person may not want their data to be held much less purchased to begin with, meaning it's "doesn't sell your data... but does or facilitates all of the things you do not want a group, in buying it from them, able to do."

It's a distinction without much practical difference.

Also: They buy your data from other brokers who do sell it, vastly enriching the degree to which customers of their ad platforms can make use of the data you already know they have far, far beyond your ability to know their full capabilities and the profile they have on you.

Again, it's not actually selling your data, but it's worth noting that when "they didn't believe it", that misconception was possibly helped along by Facebook or Google being on of the potential customers for that data either directly or via the proxy of a data broker whose largest customers are companies like that.