Want to Buy a Scrunchie Mask? Great. But Forget About That N95.

“Amazon changes the rules all the time without explanation,” Mr. Atkinson said. “I realize they’re not a charity, but a small company like ours doesn’t stand a chance.”

James Thomson, a former Amazon executive who is the chief strategy officer at the marketing consultancy Buy Box Experts, said the tensions between online retailing giants and small mask-makers reflected the larger debate over the outsize power of the online platforms that dominated the retailing landscape. Mr. Thomson, whose company helps brands navigate Amazon’s complex sales policies, said its seemingly contradictory approach to N95 masks — claiming such wares are reserved for medical personnel but then allowing exceptions for masks they have purchased in bulk — is probably an outgrowth of Amazon’s bedrock strategy for maintaining consumer loyalty.

“Even if they make next to no money on that mask, it’s really about keeping customers happy so they don’t go somewhere else,” Mr. Thomson said. “The problem is, if they allow these practices to scale up, it becomes disruptive to anything else that isn’t Amazon.”

It is hard to overstate the selling power of the tech giants. Max Bock-Aronson, a co-founder of Breathe99, a start-up in Minnesota whose washable face mask filters out 99.6 percent of microscopic particles, said his company had been ailing since Facebook dropped its ads in December, leading to a 50 percent drop in sales. “Because of our cash flow crunch, we can only make small batches of masks but they sell out immediately,” Mr. Bock-Aronson said.

He says he is especially irked by the company’s claims about needing to protect the public given Facebook’s reluctance to tackle misinformation surrounding political and pandemic-related content on its platforms.

“It’s just frustrating because we’re waving our hands, saying ‘Hey, we have a better mask that can protect people,’ but we’re really not allowed to talk about it on their site,” he said. “It’s hard enough to start a business in normal times, but it’s next to impossible when these companies shut you out of the marketplace.”

In statements, Facebook, Google and Amazon said they had no immediate plans to revise their policies.