Prompted by an article I read in The Washington Post the other day, I went to eBay recently and typed "gift card" into the search box. Presto -- 3,083 listings! What's going on? Well, the trend in recent years to present friends and loved ones with gift cards on birthdays and holidays has not gone away. Gift cards are still big business. The National Retail Federation, for example, has projected that more than $18 billion would be spent on gift cards during this holiday season.

But gift cards are not perfect solutions to gift needs. The gift card you receive may be to a store you seldom shop at or don't like. Are you stuck with a piece of plastic that may be useful only for scraping ice off your windshield in a pinch? Not anymore. There's a secondary market for such items. Remember my visit to eBay? You can now sell your gift cards, and even swap them for others. Here are a few listings I found:

  • A $20 Applebee's Restaurant (NASDAQ:APPB) gift card, on which 16 people had bid. With less than a minute remaining, the top bid was $18.02. That looks like a bargain, but the seller is charging $2.00 for shipping and handling, meaning that the winning bidder would be better off just going to Applebee's and spending $20 on his or her own.

  • A $50 Gap (NYSE:GPS) gift card, going for $46.53 with a minute remaining. Shipping was free, so this looked like it might end up being a bargain. (By the way, you're looking at "arbitrage" opportunities here. If you know something is worth $50 in one market and you can buy it for $45 somewhere else, you'll likely be able to profit by seizing the opportunity.)

  • A $200 Staples (NASDAQ:SPLS) gift card was selling for $165.22, with free shipping and five minutes remaining, and a $300 J.C. Penney (NYSE:JCP) card was going for $237.85 ($3.99 shipping) with 24 minutes left.

This isn't exactly a brand-new phenomenon -- it's just grown. (Dayana Yochim reported on it last year, in "Get Cash for Dud Gift Cards.") The Post noted that gift-card sales were expected to be up 6.6% this holiday season over last year's record high. It added, "Total gift-card sales this year are expected to reach $66 billion, according to Giftcertificates.com."

And interestingly, "... a report by TowerGroup, a research and consulting firm, estimated that 12% to 14% of those cards are likely to go unredeemed. That has created a growing 'aftermarket' for the cards ..." It's also an intriguing little tidbit for us investors. If 12% to 14% of them are going unredeemed, that translates into pure gravy on corporate income sheets. Even if the aftermarket grows and reduces those percentages, gift cards are still likely to fatten many companies' bottom lines.

If you're suddenly excited about jumping into this aftermarket, be careful. One danger is that you can't be completely sure about what you're buying. If the Gap cashier tells you that the card has expired or has only $1.13 on it, you've got a problem on your hands.

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And by the way, Gap stock was recommended in our MotleyFoolStock Advisor newsletter. Take advantage of a free trial of it to see what else has been recommended and how well they've done. (Hint: Our newsletters have a pretty impressive track record.) Oh, and Stock Advisor highlighted eBay, too, 150% ago.

Longtime Fool contributor Selena Maranjian owns shares of eBay.