Yes, there are lots of counterfeit N95 masks sold and products that are subpar. Most of the issues are from marketplace websites. Marketplace websites allow different sellers to list their products on those websites without ever seeing the product or verifying the seller. The CDC has issued guidance on identifying counterfeit respirators. The FDA has issued warnings related to marketplace sites.
The most important consideration is who you are buying the product from.
Here are suggestions to show you what should concern you when buying N95 respirators.
Besides all the basics, use a credit card – not a debit card, or use PayPal, only use a site that is SSL there are other things you can do.
The simplest thing to do is see if the website is hidden by a proxy service. There are legitimate reasons why folks do this if you are soon to be coming to market, you may not what your competition to know. But it is often abused.
Simply go to a whois directory and see if the company is listed.
For a more in-depth vetting:
Always be aware of marketplace sites!