The Time Traveler summed it up quite well above. The scammers' MO is to get you all excited about this huge sum of money coming your way and make you lose all sense of reason. Then, when you're hooked and are saying to yourself, "Great! What do I have to do to make this deal happen?", the scammers are magically reading your mind and slip in an, "Oh, we just need you to pay this tax that the Mexican Government requires you, the seller, to pay before we release the funds to you."
At this point, most victims are thinking, "No problem! I don't want to let this comparitavely meager fee get in the way of my big pay day. Tell me where to send the funds ASAP so as to make this happen."
But, considering you've been on these boards for nearly five years, you've probably read the many similar horror stories that pop up here on TUG almost weekly outlining how people have either fallen for this scam or how they have been contacted by such scammers (and the lines the scammers use).