If you are unable to make a flight or keep a pre-paid hotel reservation, three websites offer you the chance to recoup your costs and give deal-seekers a place to find bargain.

Whether it’s due to an illness or a similar unexpected event, travellers sometimes have to cancel trips on short notice – a truth that can be financially painful if the flight, hotel room or vacation rental is non-refundable.

Every year, travellers worldwide throw away about $10 billion in pre-paid hotel bookings, and $7 billion in unused flights. Thankfully, in the past 12 months, three secondary market websites – similar to Craigslist, eBay and StubHub -- have debuted services that take some of the risk out of purchasing.

These sites aren’t just places for people hoping to recover some of the costs of trips they can’t take. Vacationers looking for travel deals will also find the sites useful because the sites typically offer dramatic discounts off other people’s flights or hotel reservations.

For instance, a two-night stay at the Sheraton Chicago Hotel and Towers was recently listed on the secondary market Cancelon for $216 a night, a 31% discount off the original pre-paid price of $316 a night. A savvy shopper could even bid less than $216 a night and possibly have the seller accept that lower rate. The name on the hotel reservation would then be transferred to the buyer, and Cancelon would hold the payment in escrow until the hotel stay was completed.

A second life for secondary markets
The concept of using online exchanges to resell non-refundable travel isn’t new. For about a decade, US listings sites Red Week and Timeshare Users Group have enabled timeshare owners to rent out their properties during weeks when they would otherwise remain vacant.

More recently, sites like Airbnb, HouseTrip and Travelmob have encouraged ordinary people to rent their homes – and sometimes merely single rooms within their homes – by the night.

But selling unused flights and hotels is new – and a tad trickier. Because few people know about these young sites, they can sometimes feel like empty street markets. They’ve yet to attract a large number of buyers and sellers, a dynamic that can be self-perpetuating.

It’s best to think of them as outlet stores for vacations and to dip into them for occasional bargain finds.

Similarly, some hotels and airlines issue reservations that are “non-transferable”, which means you can’t change the name on the reservation after purchase – a restriction that, in turn, limits the unused trips that can be put up for sale.

So before you sell, you’ll have to call the place that sold your hotel reservation or plane ticket to make sure it permits you to transfer the name on the booking.

Here’s a look at the pros and cons of this new crop of sites, which will soon be joined by the Israeli start-up Roomer.

Hall St
Spanish start-up Hall St, which enables travellers to sell hotel bookings bought anywhere, is free for both buyers and sellers to post or view listings. Buyers who bid on a room will know if their bid has been accepted within 24 hours, and can also cancel their bids up to the point that the seller responds. The seven-month-old company takes care of transferring the name on the booking and if the front desk doesn’t accept the reservation, Hall St will refund the buyer’s money.

If the bid is successful, the site charges buyers a service fee depending on the type of hotel, such as three euro per night for a three-star hotel and four euro for a four-star. The seller pays a fee to Cancelon of 10% of the rate sold, unless he or she sold a room that was purchased from Hall St’s booking engine, in which case there’s no fee.

As of last week, most of Hall St’s listings were for European hotels, mainly in Barcelona and London, though the site is open to postings worldwide. Rates are per room, with a maximum occupancy of two people, so it’s not a site for group-travel deals.

When using this Israeli start-up, buyers search listings of unused hotel reservations around the world that are usually 50% off their original rate. In a unique twist, users can sign up for free email alerts when reservations are posted for their favourite destinations. The year-old company has the seller contact the travel agency or hotel to switch the name on the reservation, then the start-up forwards those updated details to the buyer. Like Hall St, it guarantees the reservation is legit.

Rates can be for multiple nights or for large rooms, adding variety to the inventory but also complexity in shopping. The site is also free to join. Sellers pay a service fee of 10% of what the buyer pays, and there’s no fee for buyers.

The Spanish start-up ChangeYourFlight enables travellers to make requests for partial refunds of plane tickets they know they are not going to use via a free, three-step process. Travellers simply enter the booking details for the journey they can’t use, request a refund and then wait to receive a voucher from the airline redeemable toward a future plane ticket. In other words, the company sells back a customer’s ticket to the airline, and the airline gives the customer a credit – typically worth half the original value of the ticket – without any charge. (There’s no market for buyers here.)

As a downside, Air One, Alitalia’s low-cost subsidiary, is the only airline to work with the start-up so far to automate the refund delivery process, and merely a few hundred passengers have received refunds in the site’s seven-month history. The start-up is eager to add more European airlines, but other carriers worldwide may be too fond of their existing high fees for cancelling tickets to consider partnering with ChangeYourFlight.

You could, of course, contact an airline directly to receive your money back, usually incurring a hefty cancellation fee. You can’t, however, generally get money back on non-refundable tickets. ChangeYourFlight provides an alternative for these non-refundable tickets, with airlines providing a partial refund without any change fee. The start-up collects a fee from the airline for creating a fresh market out of non-refundable tickets, which would otherwise be thrown away.

Sean O’Neill is the travel tech columnist for BBC Travel

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