Where are you going to go to sell it? are you going to go find some tor hidden service with a forum that you can post your exploit on and hope that somebody will give you some bitcoins for it? You think that those are not under heavy surveillance already? The "black market" for this kind of thing is way overblown. And if you think you can just go sell it to some nation state, think again. That's an easy way to end up in a federal supermax.

Sorry, I didn't write clearly. What I meant was, there is no market for RCEs in random Facebook backoffice sites.