In most cases it goes like this:
1) I collect money from buyer
2) I ask seller for pin (video)
3) I try applying pin to buyers account
4) I pay the seller what we arranged
5) I ask both parties for feedback
In the event that the pin does not work I show the seller the video, if he logs out of msn or something I report a scammer + refund the buyer.
Hope this helps clear things up.