Couch on Craiglist

A young woman, let's call her Bianca, is moving out of town. She needs to sell some of her things, get rid of them and make some money at the same time. And she does need some money. She posts an ad on craiglist announcing that she has a velvet couch in good condition for sale. Shortly she receives an e-mail from a buyer who signs as Rev., let's call her Rev. Mary. They agree on a price of $120, and the buyer sending a check. When the check arrives, it is for an outrageous amount of $1,800, and is signed by a woman not called Mary. Confused, and being an honest person, Bianca writes her buyer about the confusion. Rev. Mary replies that it is fine if the check comes from X bank. And it does. She says Bianca should cash it, take $380 for her couch (and troubles) and Western Union the remaining $1,420 to a friend of Rev. Mary's in London.

What make you of this story my friends? What kind of shady scam is it?
What should the good Bianca do?