I've never had a problem in MM, and have accumulated a healthy Trust rating, so I count myself as one of the lucky ones. However, as a spectator I have seen some proper scams and deals go west over the years.
Building Trust is the thing, and new members have to start at the bottom. With the new rules (and if people continue to follow the guidelines in the sticky regarding conducting deals) the scenario where the buyer demands the goods sent first because the seller has no Trust rating should be a thing of the past. I've never agreed with that, although I can see why it happens.
Lastly, I've had some real bargains from the MM and will continue to sell my old or replaced kit in there. Long may it continue.