Facebook doesn't sell data. They sell access to data. I'm not trying to be pedantic, but it's important because it's meaningfully different.
Facebook doesn't sell data to other ad companies because that'd be giving away the very thing that makes it the most valuable advertising company in the world. Hell, we spend a huge amount on facebook, and it's impossible for us to get reports on basic things like "Spend per placement per hour".
The only area that I could see them _maybe_ still monetizing an opt'ed out user is with Meta Audience Network, but I bet they would still opt them out of seeing ads on 3rd party sites that have MAN ads in their stack because that business segment is likely small compared to potential PR and fines in the EU.