You just realized the booking agency gave you a seat by the toilets on your next flight, and there are no open seats you can swap out. Ugh. A new app called Seateroo could make the difference between three hours of your life spent pinching your nose, and a pleasant in-flight nap. Seateroo (free on iOS) lets you buy or sell seats on your flight directly with other passengers aboard the same plane. The app is not affiliated with any airlines — you're buying and selling passenger to passenger. Thus, you're not actually selling your ticket, and you're also not changing the seat assignment listed on your ticket.Here's how it works: When you open the app, you search your flight number, and then it shows any available seats for sale. If you ended up with a sweet window seat but you don't actually care where you sit, you can also choose to sell your own. Just enter the seat information, including the seat number and whether it's row, middle, or aisle, and set a price. Prices for all seats start at $5 (nope, you can't swap seats for free), and the app takes a 15% cut. You can search your flight up to five days before it takes off, but the actual swapping happens once you board, or after takeoff.So, couldn't you just do this on your own, without paying any money? Sure — you've just got to ask the fellow passengers milling around your gate if anyone wants to seat swap with you. I've never yet been that inclined to change seats.But with this app, you could have a convenient way to make a quick buck from someone itching for some extra leg room, or a chance to make your flight experience more pleasant. That is, as long as the seller actually follows through with their promise. If for some reason your flight is canceled or the seller decides they don't want to change seats after all, you can complain to Seateroo (not the airline's flight attendants) for a refund, but it's not clear if you'll actually get your money back.Seateroo is U.S.-only for now, and you'll need a credit card to pay (although Apple Pay and PayPal integration may be coming soon). We'll definitely give it a try next time we find ourselves pigeonholed among a gang of wailing babies.