Costco has been selling Birkenstocks for as little as one-third of the normal retail price in some stores.

However, Birkenstock said that it does not sell to Costco and that Costco isn't authorized to sell its sandals.

Birkenstock Americas CEO David Kahan told Business Insider in an email that the company was ready to "take legal action if necessary to protect our brand."

Costco did not return Business Insider's request for comment.

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In late February, Costco fans were thrilled to find Birkenstocks for $60 on the Costco website — nearly half the price listed for the same style on Birkenstock's website.

And on Tuesday, an Instagram user unaffiliated with Costco posted a picture of Birkenstock's Arizona sandals apparently being sold for $30 at a Bay Area Costco. The same style is sold on Birkenstock's website for $135.

If that deal seems too good to be true, that's because it is.

Birkenstock Americas CEO David Kahan told Business Insider in an email that Costco is not an authorized retail partner of Birkenstock. It is unclear whether the Birkenstocks sold by Costco are authentic, and if they are, how they were obtained.

"We do not sell to Costco," Kahan said. "While we can't comment on whether the product is real or counterfeit, be assured we take any unauthorized selling very seriously and all steps are in place to control our distribution and take legal action if necessary to protect our brand."

So-called gray markets, in which retailers sell authentic branded products without the consent of the brand, have long been a thorn in the side of popular brands.

A Costco representative did not respond to Business Insider's request for comment.