WASHINGTON — Scammers are peddling SmarTrip cards near Metro stations and online, and some deal-hungry buyers are left with useless plastic once cards are flagged for deactivation.

If deactivated, the cards from a secondhand seller have no value.

Metro on Monday issued a warning to riders about a fare-card scam that has intensified in recent weeks.

“Really, if you’re buying a SmarTrip card with value on it that’s higher than what you pay for it, it’s buyer beware,” says Metro Transit Police Chief Ronald Pavlik.

“We have the ability to deactivate those cards, so you’re out the money you paid for the card and then whatever value might be on the card,” he says.

Metro says it investigates potentially fraudulent sales on the streets as well as on online auction sites.

Listings this month alone on Craigslist offer SmarTrip cards with values ranging from $42.10 to $2,700.

Metro did not provide figures for how many SmarTrip cards are deactivated. All cards purchased from non-official sellers are considered invalid and may be flagged for deactivation.

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