A Presto card scam is prompting Metrolinx to urge customers to only buy cards from authorized sellers.

The transit agency was alerted to the scam by a customer who nearly lost $500 last week, spokesperson Anne Marie Aikins said.

Here's how it works: the scammer offers to sell a pre-loaded Presto card — Aitkins said the amount has ranged from $50 to $500 — for less than that amount. The scammer and victim will meet somewhere like Union Station, where the amount can be verified, then exchange cash.

But before the person who bought the Presto card can use it, the seller, who has previously registered the card, cancels it, rendering it useless.

"It may seem like a good deal, but at the end of the day you're going to lose your money," Aikins told CBC News.

It's unclear how many people have fallen for the scam, but Aikins said Metrolinx has seen a number of these offers popping up online. Metrolinx is urging people to only buy Presto cards from its stations or other authorized sellers to avoid getting ripped off.

"Even one person scammed is too many," Aikins said.

Anyone who has been victimized by the scam should contact police.