Don't know if this is still the case, but in the past it was possible to pay facebook for ads targeting e.g. 25-30 year olds in the US, and color that add with a cookie, so that when you saw the same browser later, you'd know it was a 25-30 year old from the US.

(It went much deeper than that - marital status, month of year, a few other things I can't remember). So, while they weren't directly offering and selling this information as is, you could buy it from them by buying ad space on the demographics you cared about. (Yes, this also required you had other access to ad networks, real time exchanges, etc -- but if you really want info on people, it was chump change)

Facebook "partners" with several large 3rd party advertisers.

I'm not sure how you would define "sell data", but other advertisers are using that data for targeting at scale already.