which got me thinking if the cards they are selling are genuine refurbished cards or counterfeit?

It depends on the seller.

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They could also be legitimately produced cards that didn't meet QA standards and were supposed to be recycled but someone stole them to resell.

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They could also be "ghost shift" cards, which is a whole category of counterfeits:

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a) Extra capacity produced by a vendor off the books. (E.g. Company A orders Company B to make 10,000 widgets. Company B makes Company A their 10,000 widgets, but secretly makes an extra 5,000 widgets to sell themselves).

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These are "legitimate" cards insofar as they were made by the same sub-vendor using the same machines, materials, and processes, but they are not officially licensed. They may even be QA tested.

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b) Produced outside of normal business hours. (E.g. "Bob" snuck into the factory at midnight and turned on the widget machine for an hour and took them home to sell).

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Again, these are "legitimate" cards insofar as they were made by the same vendor using the same machines, materials, and processes, but they are not officially licensed, and almost certainly did not go through QA testing.

(The main difference between this and the above is usually scale/scope. In the first case it's a company intentionally overproducing vs this is one or more people doing it unknown to the company).

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c) Produced with inferior quality materials. This could be a case of a) or b) but while also using lower/cheaper quality materials to make even more money. These may still work, but are very likely out of spec and more prone to failure than the above.