Though Shopify has been policing the new sites, it also encourages its customers to go into the dropshipping business. It offers a guide for starting such a business and makes money from them by charging a monthly fee and a percentage of sales. The Canadian company is one of the largest turnkey e-commerce sites in the world, bringing in $1.5 billion last year. In February, Shopify announced that it had hosted over a million businesses.

New sites selling coronavirus products come online every day. A majority of the sites tracked by The Times appeared over the past two weeks, including over 70 registered since Wednesday, according to data from DomainTools, a cyberforensics company. The sites target users around the world and are in English, French, Spanish, German, Romanian, Icelandic and other languages.

The registrations are part of a larger increase in Coronavirus-related activity, said Chad Anderson, senior security researcher at DomainTools. In February, he said, he saw about 100 new websites a day related to the pandemic. Now, the figure is 2,000. Their systems have flagged about half of those as likely related to malware, ransomware or phishing.

“It’s a massive uptick,” Mr. Anderson said.

The operator of another Shopify dropshipping site, covid-defender.com, who would identify himself to The Times only as Radwan, said he lived in Denmark and had run Facebook ads for his site. The site sells face masks for $30 to $40, including one marketed for children described as “Kid Mask Protection Against Virus and Bacteria With N95 Standards.”

He said he believed the supplier’s statements that the masks were certified to the standards claimed. “It doesn’t say by any means that it provides 100 percent protection,” he said.

He said he had not heard of shortages of the masks he sold. “If I had heard of any shortage anywhere on the planet I would not sell it,” he said. Even so, he said, it was up to his customers to vet the products before buying.

“They shouldn’t trust these stores, and they have to find the proper information themselves,” he said.