It is an open anonymous marketplace and these issues go hand in had with navigating SR. The admins are however pretty good at investigating this type of behavior when it is brought to their attention and taking the appropriate action. Whenever you see anything a bit suspicious from a vendors behavior, be that a sudden change in comms, drop in price of certain listings, asking for FE when they haven't in the past etc then flag it, discuss it and bring it to the broader communities attention. I can spot a scam happening a week before it gets lots of attention of here, the main issue is it can be a tough call to make sometimes and the admins need evidence which sometimes means the scam has to have been running for a little while before that can call it and act. MP for BTC is one of the most prolific scams running right now so if you can find another way to buy your bitcoins please do it because I see no end to this type of scam anytime soon.