But over the past decade, airlines have transformed the way they fill their premium cabins, replacing complimentary upgrades with everything from sealed auctions and live bidding wars to instant buy offers and loyalty point competitions. Now, dozens of airlines including Abu Dhabi's Etihad Airways run their own marketplaces where thousands of premium seats change hands every day. If your bid succeeds, you’ll receive a new boarding pass – if it doesn’t, you pay nothing. Behind that seemingly straightforward offer, however, sits one of the travel industry’s fastest-growing revenue streams. A business class seat, after all, is one of an airline’s most valuable assets, but only until the aircraft doors close – and once a flight departs, every unsold seat becomes worthless.