Call it frequent flier envy.

Though frequent flier perks are growing fewer and farther between these days, there is one perk that remains for some: the upgrade certificate.

But what about those travelers who aren’t willing to play the mileage game — the folks who aren’t interested in airline loyalty, who refuse to pay for their kids’ educations with a credit card to rack up a few miles?

But wait — there seems to be a loophole for the anti-frequent flier: You go online and find that a traveler is auctioning off a certificate that promises you a free upgrade on your next flight. This, too, is an upgrade certificate — but it can also be a lesson of buyer beware.

Traditionally, an upgrade certificate is a perk that’s mailed out on occasion to frequent-flier program members, or is part of an incentive package to sign up for a credit card.

In theory, it sounds like a golden ticket: a free upgrade on your next flight, launching you from the back of the plane to business or first class.

So when many of us see the chance to purchase an upgrade certificate on eBay, Craigslist, or even more travel-oriented sites like Hotwire and Priceline, it’s certainly a temptation.

But talk to most frequent travelers, and you’ll find that these certificates often end up in a pile in a drawer collecting dust… for the same reasons that there are 9.7 trillion unredeemed frequent flyer miles in the U.S.

Just like frequent-flier miles, upgrade certificates come with a planeload of restrictions and disclaimers. For example, you can only book the upgrade based on availability within 48-72 hours of the flight, the upgrade might only apply to full coach fares, and blackout dates almost always apply.

It’s also important to note that even if you have an upgrade certificate for one airline, if your flight is actually being operated by another carrier, it may be invalid on any of these codeshares.

Keep in mind that there is no one type of upgrade certificate: United, for example, has a traditional paper upgrade certificate which might cover a one-segment upgrade that applies to only one flight number; a one-way upgrade that applies to all distances, legs and connections; a one-way distance upgrade that is based on how far you’re traveling, a standby upgrade… you get the point.

Your type of ticket can also dictate whether you can even be upgraded at all; for example, a deeply discounted, non-refundable ticket probably isn’t eligible for the upgrade, no matter what kind of certificate you’re holding in your hand.

But what about purchasing an upgrade certificate through a third party?

The basic rule seems to be this: When it comes to any perks, whether it’s an upgrade certificate or free mileage, it officially constitutes a violation of the mileage agreement for anyone to sell it. . But while airlines only crack down infrequently, remember, you’ve been warned.

Therein lies the technicality. If you can’t buy or sell one, you CAN give or receive one as a gift to/from family and friends.

Just make sure it’s a gift — based on your type of ticket — you can actually use.

Check out Bill of Rights for Air Passengers, and Why I Hate British Airways for more reasons to travel first class (at least you’ll have a comfortable seat if you’re stranded on a grounded plane for a few hours).