Some eBay entreprenuers are making $20 to $30 selling a coupon they (and probably you) got in the mail for FREE.
ValPak sends out a blue envelope every month full of coupons for house siding, haircuts, oil changes, etc.
In the current mailing in some regions of the USA they are including a Northwest Airlines/KLM coupon voucher for a $100 USD discount on roundtrip travel in the continental US and Canada. The coupon has no black out dates, no limited seating restrictions and no 7-day advance booking requirement.
Some entrepreneurs realized the value of the coupon and quickly posted listings on the online auction site.
Moral to the story: Look at your junk mail a little closer.