A pk is a $ off coupon that they usually give as perks for missed flights, bumps, etc.

What I don't understand is why NW doesn't shut down sites & auctions that rampantly sell the PKs. They make it so easy to just paypal 12$ and get like 100$ off your flight its a wonder that more people don't do it.

Seems like restricting PK distribution would shut down 75% of the traffic.