I don't doubt some people use it as a legitimate venue to sell odds & ends, but it doesn't take long to stumble across clearly forged goods and people selling animals (the second type I reported as it's against FB rules and basically not really on either. Not that forged goods are ok either, but I hope you get my drift). If it was all just really poor attempts at "craft" I could turn a blind eye, but NO-ONE has 25 authentic Michael Kors bags that they are happy to sell for £15, do they? Surely forged goods aren't just being sold for extra cash for an individual, they are often part of a dark underbelly of drugs/trafficking/funding illegal activities?
On a non-illegal matter - today one of the latest posts is someone selling used sex toys. I know they are used as they say "would love someone to have the fun I have had with these!" 
- though this probably just contravenes my sense of taste and decency!
I still haven't worked out if there's a way to turn the Marketplace thing off, considering FB don't do anything to ensure people abide by their selling rules.
I realise this may come across as me being all offended, but I'm genuinely surprised that FB aren't willing to police this stuff. It's not like they don't know who is posting it!