Suppose someone's profession is not selling stuff and just using Facebook Marketplace for selling items not needed any more, Is it OK to refuse selling to particular person or block a particular person from buying on Facebook Marketplace?(just like blocking people on WhatsApp/Facebook/Google)
If you are having a garage sale or selling items one at a time the same way, just things you own that you no longer want, of course you can sell to whoever you want and refuse to sell to people for any reason. Maybe they gave you a bad check last time or wanted a refund. Maybe you just don't like them. Facebook doesn't have rules requiring you to sell to anyone who asks, or whoever gives you the best price.