According to Waldstein, founder and principal of Apex Compliance, there have been only four eBay-related warning letters in recent years–two of which occurred recently.

"What is notable about this warning letter is that the FDA does not appear to have purchased or tested the product, which is often the case with more shady-looking weight-loss product," he said.

Instead, the apparent basis for the warning letter is the drug-related claims made for the product.

"Usually it has to do with more disease claims, so it might be GLP-1, blood sugar support and then maybe a claim in a hashtag or a claim in social media," he said. "But usually these types of claims are not enough for a warning letter. They usually are enough for FDA to test the product. And in this instance, FDA actually didn't test the product. They said GLP-1, blood sugar support claims–that's enough for a warning letter. So that was significant."