Travelers who have tried to redeem frequent flier miles for award seats, only to be thwarted by availability, know that attractive alternatives are hard to come by.

You could spend miles for magazine subscriptions, donate them to charity or buy merchandise in an uneven exchange (17,000 miles for a coffee maker?).

Now imagine a virtual stock market for the eBay generation where the miles and points are currency, and the free market — rather than the airlines — determines the exchange rates. That's the general idea behind some new online services.

Such mileage matchmaking services have popped up as airlines, including American, Continental, Delta and United, have been announcing significant capacity cutbacks — reductions that are expected to make frequent flier awards even harder to use this fall.

One site, Points.com, has introduced a service called Global Points Exchange that allows travelers to barter miles with one another and set their own exchange ratios (I'll give you 8,000 American miles for 10,000 of your Delta miles).

For Andy Simmons, a software solutions consultant from Chicago, trading was worth it.

"I was stuck with these 16,000 Delta miles I'd probably never use unless I bought more, so I had to make an economic decision," he said. Simmons made three trades — two at an even swap of 4,000 miles each and another trading 8,000 of his Delta miles for 7,000 American miles. Total transaction fees: $250.

Another site that is letting travelers set their own trading terms is LoyaltyMatch.com, which started in February. It's the brainchild of Brad Ball, a technology executive who had accumulated piles of miles in business travel.

Like so many other fliers, Ball was having trouble using them for something he really wanted.

LoyaltyMatch, based in Waterloo, Ontario, offers an alternative, he said, by letting travelers convert those unused points and miles into merchandise that they can then sell online for cash or swap for other items, services and activities.

Sellers register and enter their loyalty programs and account totals, which LoyaltyMatch uses to automatically list the items those miles and points can buy. For example, my test account (with 16,309 American Airlines miles and 15,000 American Express points) immediately triggered four Exchange Alerts from other users willing to pay cash or trade miles or points for the merchandise that LoyaltyMatch said my miles and points could buy.

One user from Canada was willing to pay $35 for a Cuisinart popcorn maker that would cost me 10,000 AmEx points. Another proposed a trade for a Wired magazine subscription that 400 American Airlines miles would buy.

Buyers simply browse the site for items they want and submit a cash or trade offer. The site then matches you up with potential sellers, who can choose to accept your offer and use their points or miles to buy that popcorn maker for you and have it mailed to your door. You in turn, pay the seller through PayPal or order the item you agreed to trade from your loyalty program. For its services, LoyaltyMatch charges $1.99 a transaction.

While LoyaltyMatch.com and Points.com seem to hold a lot of promise, they still have technological glitches, and they are bound by the rules and regulations of the loyalty programs themselves.

At Points.com, along with a $6.95 transaction charge, you still have to pay the airlines' traditional mileage-transfer fees. Delta, for instance, charges a $30 transfer fee and 1 cent per mile. American charges $80 for up to 5,000 miles, $130 for 6,000 to 10,000 miles, and $180 for 11,000 to 15,000 miles. Points.com also has limited trading options. Alaska, American, Delta, Frontier, IcelandAir and Aeroplan, the flier program spun off by Air Canada, are the only participants so far.

A big drawback of LoyaltyMatch is that not many of the major domestic airlines have merchandise programs. So a traveler looking to unload American miles, for example, can really only sell magazines for cash.

If all you really want is a "free" ticket, check ExpertFlyer.com, which charges $4.99 a month for basic service. It lets frequent fliers see how many award tickets or upgrades are available for a given flight by listing airline fare codes.

Travelers searching for tickets can also set up an alert to receive e-mail when award seats are released for a specific flight.