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It happened to me the first time a few weeks ago outside the Fort Totten Metro station. Two teenagers walked up, pulled out five or six Metrochek fare cards and offered each for half their $20 face value.

If I hadn’t already paid for a monthly pass, I might have bargained. Instead, as they walked away, I wondered how a teenager wound up with more than $100 in Metro tender, and why I was paying full price.

In April, the GAO reported that Washingtonians were defrauding the federal government of at least $17 million a year by selling their Metrocheks for cash, often on Internet auction sites. Trading cash for Metro fare cards which many federal employees receive as part of their benefit packages, is illegal, even if your son peddles them at Metro stations.

But knowing about and shutting down the taxpayer-funded fare black market are two different things.

The sales also continue on the Internet. Today on eBay, there were three options for reducing your commuting costs.

But the market could be coming to an end. Beginning in January, WMATA will transfer all federal and private employers to SmartBenefits, a Web-based program that will allow employers to drop cash on workers’ SmartTrip cards.

Then again, how much would you pay for a plastic card that magically adds $20 in fares a week?