It would be much easier for authorities to track down participants in that marketplace. It requires them caring and following through. When it comes to antiquities these things can be resold often, so even if there is a case for seizure when found, the liability isn't always there to pursue someone.

Maybe authorities not caring should be the headline.

Actually, it's a statistical certainty that a certain number of illicit transactions take place on Facebook Marketplace. If we were writing a program that we knew would have a bug in a certain number of cases then we would write a test for it or write exceptions. If Facebook is handling this responsibly then I'm sure they have a process for identifing wrong do-ers and handing everything they know to the authorities.

When they figure a way to monetize it, they will sure do it. But then we all will end up in jail.

if ( Facebook.HandlesResponsibly(“black market”) == TRUE ) then me.colour = “surprised”

Facebook isn't selling data, it's their most valuable asset, that'd be unbelievably stupid. They're renting out your attention span.