By Jon Emont

Questionable online sales tactics used by some foreign merchants have been brought into focus after the federal government said this week that mysterious seed packages arriving from China may be an e-commerce scam.

The U.S. Agriculture Department said that "suspicious, unsolicited packages of seed that appear to be coming from China" are potentially part of a brushing scheme. In such a scheme, sellers or their accomplices ship out packages containing lightweight goods of trivial value, which can then be used to log fake sales and generate fake positive reviews that can draw in more customers and boost sales.

Over the last decade, online marketplaces like Amazon.com Inc. and others have signed-up Chinese manufacturers and merchants that sell products directly to Americans. Some e-commerce sellers and experts have linked these sellers to dubious sales tactics, like brushing schemes, on the platforms.

E-commerce sellers and merchants based in China said that while seeds aren't known to be used to fill packages in brushing schemes, they are lightweight and cheap and can be easily mailed to hundreds of people.

The seed packages have attracted the attention of government officials world-wide. Some, including state and federal officials in the U.S., have asked those who receive unsolicited packages not to plant the seeds out of fear they could introduce invasive species or spread plant disease.

Howard Thai, the Shenzhen, China-based head of Signalytics, an e-commerce consulting firm, said that while he didn't know who sent the seeds, it was possible the organizer of any scheme had miscalculated and was unaware that seeds would cause alarm. "If you put sand in there it's different," he said. "The people doing it are not that smart."

Mr. Thai said that those doing brushing schemes will often send items they hope the recipient will enjoy, to prevent those receiving the packages from complaining about receiving a random package.

E-commerce experts and sellers in China say that an entire ecosystem has sprung up to assist Chinese sellers who are seeking positive reviews for their products as a way to boost sales. On Weibo, China's Twitter-like service, e-commerce merchants connect with those willing to provide reviews through the hashtag "Amazon Review," seeking out people with American addresses who can provide "real person" reviews.

On another website, amz123.com, Chinese posters provide evaluations of brokers and individuals they have paid to give positive reviews for their Amazon products, sometimes complaining that their product didn't receive the reviews they were promised. Weibo and Amz123.com didn't immediately respond to requests for comment.

Amazon has said the seed packages didn't appear to be part of a brushing scheme. "These appear to be delayed packages due to Covid-19, not brushing," a spokesperson said in a statement Wednesday. Amazon didn't respond to a request for comment Friday.

Many seed recipients say they didn't order any seeds, according to state agricultural officials. The Wall Street Journal spoke with several recipients of mysterious seeds, some of whom said they got unsolicited seeds after ordering and receiving other seeds, or had different seeds on their Amazon wish lists. Still others found small packets of seeds in packages of other goods they ordered, such as clothing.

One resident of Washington interviewed by the Journal said she had received corn, broccoli and cucumber seeds she had ordered from Amazon earlier this year, but that last week a package had arrived from China with small cream-colored seeds that made her nervous and want to burn them.

Western e-commerce companies have for years sought to recruit Chinese merchants to their platforms, as a way to boost product variety and provide cheaper products, though there have been challenges, including counterfeit products.

In 2017, one frustrated Chinese seller emailed a senior Amazon executive, writing, "Many companies have sprung up in the last couple of years" that deliver "illegal service to businesses requiring a quick and dirty way to boost their products' popularity on Amazon," according to emails shared with the Journal. The Chinese seller wrote that algorithms alone would be unlikely to catch mischief, as "Chinese sellers are too smart and easily detect out the loophole of the Algorithm." It was unclear from the emails shared with the Journal how Amazon responded to the seller's concerns.

An email from a former Amazon senior vice president to other Amazon executives in 2015 about the company's efforts to promote sales from China mentioned there were unusual aspects to the market there. His email, released by the House Committee on the Judiciary during hearings on tech companies' market power, said there were, "Lots of competitive dynamics between sellers that make this a very different animal for us." The email said that the company was building "local TRMS" and other services to "aggressively better serve this population of sellers." TRMS refers to Transaction Risk Management Systems, the Amazon organization which builds fraud and abuse risk-management tools for the company.

In a statement last year, an Amazon spokesperson said the company was taking steps to prevent abuse in its store. "Bad actors that attempt to abuse our systems make up a tiny fraction of activity on our site. We use sophisticated tools, including machine learning, to combat them, and we are making it increasingly difficult for bad actors to hide."

China's Foreign Ministry said Tuesday that mailing labels on the seed packages were forged and that the country has asked the U.S. to return the packages to China for investigation.

Qianwei Zhang contributed to this article.

Write to Jon Emont at jonathan.emont@wsj.com