That tango class on Groupon was such a great deal! Too bad you didn't ask your sweetie if he wanted to learn to dance. Now you're out $45.

Or maybe not. A secondary market for Groupon and other social commerce deals has sprung up. Sites like Lifesta, DealsGoRound and Coup Recoup provide a way to unload unwanted vouchers.

They're also a playground for bargain-seeking consumers. Missed a screamin' deal from Groupon, Living Social, Tippr, Buy With Me or another social buying site? You might get a second chance.

You can even buy another massage voucher if you originally bought just one and then decided it would be more fun with a friend.

The resale sites cover the same categories as the originals, with vouchers for restaurants, theatres, museums and other entertainment. Need a haircut, new glasses, a dental exam, an exercise class or someone to help an elderly relative with heavy chores? You can meet these and other everyday needs cheaply through social buying.

'A very, very big market'
At least 350 daily deal sites currently operate both regionally and nationally -- and more are on the way, according to Jim Moran of deal aggregator Yipit. Such sites have sold tens of millions of vouchers in the past two years.

However, it's estimated that at least 20% of those deals never get used. Some buyers forget they have them; others wait too long and can't use them up before they expire.

Moran has been watching the secondary sites since the first one (Lifesta) opened in May 2010, and he writes about them on the Yipit blog. He predicts the coupon resale space will become "a very, very big market" because it helps two kinds of consumers: those who miss the deals and those who, for whatever reason, don't use the deals they bought.

New Yorker Polly Chung has bought "hundreds" of social commerce bargains. "My husband and I don't do anything unless it's a discount," she says. Occasionally a multiple buy doesn't work out: a spa that didn't meet her expectations, gelato in a neighbourhood she never gets around to visiting.

Chung has unloaded half a dozen such deals through Lifesta, which she calls the perfect place "to sell what I don't want and to buy what I would like more of."

Beyond impulse buying
One advantage of daily-deal marketplaces is their sheer variety. You expect to see a lot of dinners, pedicures and spas. But a quick glance also turns up bargains like:

* Two hours of work from a landscape company for $22.
* An oil change for $10.
* $50 worth of meats from a specialty butcher for $22.50.
* A class on making bath products and herbal remedies for $6.
* $40 worth of books at Simon and Schuster's online store for $20.
* An all-day pass to a paintball venue (including equipment) for $20.

Resale sites turn daily deals into "a catalog experience vs. an impulse buy," says Aren Sandersen, founder of Coup Recoup.

Don't see the one you want? At DealsGoRound you can post a "wanted" ad. For instance, Jeff Templin missed out on a Groupon deal for a bar and grill near his workplace: $35 worth of food for $15. He advertised for the voucher, found it and used it to take a co-worker out for lunch.

"I paid the $15 and he paid the tip," Templin says.

Getting started
The process varies slightly among the three sites, but it generally goes like this:

* Create an account (except at Coup Recoup, which doesn't require one)
* Post your deal (usually by uploading a .pdf file)
* Wait for someone to buy

Transactions on Lifesta and DealsGoRound are done through PayPal or Amazon Payments. Sellers pay a fee: 99 cents plus 8% of the selling price on Lifesta, 10% of the price on DealsGoRound.