Air travel "coupons" on ebay - catch?

I was doing a websearch for a United coupon, in the hopes that I could find a cheaper fare for traveling to my cousin’s wedding in a month, and I came across many ebay auctions selling $75 coupons for United.

an example.

The auctions all basically say the same thing about blackout dates and destination restrictions, but the pictures are fuzzy enough that you can’t read the small print, and who knows what’s on the back.

I know that airlines have crazy promotions and complicated fare schedules, but I find it hard to believe that they’re just throwing out 15%+ coupons valid for any continental flight. Aren’t their operating margins razor thin as it is?

Does anyone know if these are legit? Can I actually save money by spending $5 on a $75 coupon?

I would be wary. but they may work e.g. from Gems and Junk on eBay's Travel Section - March 19, 2002

"For example, I found a couple of United upgrade certificates – good for domestic and international flights – with starting bids of $75. Now, these are tough-to-get certificates are given to United’s most frequent flyers, those who fly at least 100,000 miles a year.

But do your homework before bidding. First of all, two of the certificates I found on line expire in less than two weeks – on the last day of March. Secondly, while the sellers correctly state that their coupons are valid for most fare categories, the important word here is most. If you hold a very cheap, advance-purchase ticket you may not qualify for an upgrade, even with this magic certificate. Fare categories are identified by letters, H, Q, Y – as many as a dozen of them. One of the most common advance-purchase fare categories is designated by the letter K, and it’s not up-gradable on United with one of these coupons. So, know your ticket’s fare category before you bid. Or, make sure, when you buy your coach ticket, you ask for a fare category that qualifies for an upgrade.

One other footnote: United Airlines stipulates that these coupons are void if sold. That’s why, when you click on purchase details, you’ll find the seller is selling something like – and here I quote – “one white envelope.” Ah, but in that envelope just happens to be a United, system-wide upgrade certificate – good, old American ingenuity. "

I’ve bought the Southwest vouchers twice and love them(the Rapid Rewards program)…You can make the reservation the day of the flight, change it whenever you want on the return, etc.

I typically pay 250-300 bucks for them. That price is great if you have short notice travel, or don’t know when you need to return, for example. Ebay won’t allow them on the site anymore, so you have to check for them often and email the seller when they get listed and try to buy them before they get pulled.