Sadly Facebook Marketplace seems to be the most active.

I say sadly because the overall level of intelligence, diligence and responsibility is much lower than Craigslist, eBay or forums. This of course is my anecdotal opinion.

eBay has a bias towards the buyer during disputes and many people are taking advantage of that.

Craigslist was basically sidelined here in Canada due to eBay's heavy advertising of their competitor called Kijiji.ca

A further problem here in Canada is that shipping is pretty expensive so local pickup works best.