Mike Molson Hart, who sells toys on Amazon.com Inc.’s marketplace, realized earlier this month something was amiss. His company’s popular disc-shaped plastic building set, called Brain Flakes, had dropped precipitously in the ranks of Amazon’s best-selling toys as the critical gift-giving season approached.

He visited the product page on Amazon.com and suspected he was the victim of “sniping,” when one merchant sabotages another by hiring people to leave critical reviews of their goods and then voting those reviews as being helpful, making them the most prominent feedback seen by shoppers. Freelancers in China and Bangladesh willing to do this for $10 an hour are easily found online. Even though the toy has a 4.8 star rating out of 5 based on more than 1,100 reviews, shoppers first see a string of critical one-star reviews and many may get scared away.

Maintaining order on Amazon — where 2 million merchants compete to win billions of dollars in business from 300 million shoppers — has become a running problem for the online giant and it only heats up during the busy holiday season. Some merchants engage in black-hat tactics with precise timing, trying to maximize their own sales when shoppers spend most before their tricks are detected. When Amazon clamps down on one exploit, they regroup and find a new one.

“This stuff has been going on nonstop since we started selling on Amazon,” said Hart, president of VIAHART in New York City. “It’s still the Wild West. There are tons of scams and they constantly evolve to keep gaming the system.”

Amazon said it “does not tolerate fraud or abuse of our policies.” In a statement, the company said it’s “constantly working to improve the ways we detect and prevent abuse from impacting customers.” Amazon said it suspends or blocks “bad actors” suspected of illegal behavior or infringing on others’ intellectual property rights.

The trickery escalates during the holidays when the stakes are highest. U.S. online spending in November and December will top $107 billion this year, with the five-day period between Thanksgiving and Cyber Monday the busiest, according to Adobe Systems Inc. Merchants who are targets of sniping stand to lose the most. If shoppers are dissuaded by fake reviews or ratings, they’ll just move on to another product and Amazon still gets its commission on the sale. The targeted vendors have few other options since Amazon is the world’s biggest online retailer.

Manipulation of reviews has been increasing the past several months and Amazon doesn’t appear to be fixing the problem, said Chris McCabe, a former employee who now runs a consulting business to help Amazon merchants. The gamesmanship on the site is so bad he has created new teams to help merchants fight review manipulation, he said.

“It’s a massive problem and until it’s more publicly known I don’t think they’ll do anything about it,” McCabe said. “There’s blood in the water and everyone knows they can get away with it, so it’s a free-for-all.”

The threat to Amazon is faith in its customer reviews, which it has used to boost confidence of shoppers buying something online they may not have touched or seen first-hand.

Two years ago, Amazon filed lawsuits against more than 1,000 people it alleged were writing fake product reviews for money through the online marketplace for small tasks Fiverr.com, saying they were diminishing consumers’ trust. Last year, Amazon filed lawsuits against merchants it accused of selling counterfeit products, acknowledging it couldn’t police the problem on its own.

The Seattle-based company also clamped down on “incentivized” reviews written by people who received free or discounted products. Amazon initially saw such posts as a way to help new products get discovered and required those receiving free and discounted items to make disclosures. It has since realized the practice of offering freebies for reviews was being abused.

The lawsuits and crackdowns are proving to be little more than speed bumps to those looking to game the system, who keep finding new tactics. Fake product reviews are now offered on the classified website Craigslist, with posters offering $10 through PayPal for each 5-star review. Finding a freelancer in Bangladesh to write fake reviews or up vote the negative reviews on a competitors page is as simple as visiting the online marketplace Upwork and searching “Amazon up vote.”

Product review forums on Facebook are a common place merchants can search for accomplices, sending them Amazon gift cards so they can get a free item in exchange for a positive review, while eluding detection from Amazon.

Some merchants seeking to game the system also take advantage of Amazon’s heightened sensitivity toward counterfeits. Cheaters can file a false trademark infringement claim against a competing merchant, getting a legitimate business suspended in Amazon’s guilty-until-proven-innocent vetting system. Even if the target of the false complaint gets its selling privileges reinstated, the process can take days or weeks, giving the competitor valuable time to win sales.

Another scam is to relentlessly click on the Amazon advertisements of a competitor’s product without buying anything, which drains their advertising account. Amazon issued refunds to multiple merchants this year after discovering “invalid” advertising clicks, which merchants suspect was gamesmanship by rivals.

Hart said he’s frustrated with Amazon’s response to his complaints about his products’ reviews getting manipulated. He said he fears retribution for speaking publicly, but the stakes are high as he estimates he could lose $1 million in sales this season. Hart has been going back and forth with the company via e-mail and phone calls since Monday. Amazon has said they’re examining his concerns. Meanwhile, Hart has dropped his prices and sales are still sluggish.

“Amazon knows about this problem and it has become their policy to not do anything about it,” he said.