This is what Saoud Khalifah was doing when he started to notice something strange about the products he was buying.

So I got interested in online reviews when I actually was finishing up my master's degree, and I ordered a few things off Amazon.

They were five-star rated.

There were hundreds of reviews, and they were all really, really positive.

I received the product, and there was something completely off with the products.

The quality was really low.

It did not last at all.

So I went back, read the reviews, and noticed there was a lot of red flags within the reviews themselves.

There was a huge trend of fraudulent activity within the reviews, and I could tell that they were fake.

Mr. Khalifah now runs a New York-based company called Fakespot, one of several new businesses which claim to be able to help you spot a fake review.

The very existence of such companies highlights a looming crisis of faith in online reviews, which are now at the very heart of how internet shopping works.

In the U.K. alone, they influence an estimated 23 billion pounds of transactions each year.

Most internet users are aware of the fact that they have to read reviews before doing a purchasing decision.

And without any reviews there most products won't get any sales.

That's basically it.

So most of these sellers know that reviews are a very critical part of their business.

Faced with these pressures, some companies are now encouraging customers to leave a positive review in return for a free item.

Others opt for more cynical tactics, targeting competitors with negative feedback.

In a statement, Amazon told us that "any attempt to manipulate customer reviews is strictly prohibited."

And that it suspends, bans, and takes legal action on those who violate its policies.

The U.S. firm added that it "invests significant resources to protect the integrity of reviews in our store."

This includes "teams of investigators and automated technologies to prevent and detect inauthentic reviews" at source.

The US company has filed lawsuits against more than 1,000 defendants for reviews abuse.

But Amazon is an enormous global platform for selling, and so any attempt to monitor wrongdoing comprehensively is going to be seriously difficult.

It is a cat-and-mouse game between the platforms, for example, Amazon and the sellers that are on those platforms.

So they constantly find new angles, new ways, to exploit their system and find new ways of detecting fake reviews.

So it is definitely a cat-and-mouse game.

When it comes to smaller sellers, reviews can be a matter of life or death.

Kevin Williams founded Brush Hero, a company that sells brushes used to clean cars.

Reviews have an incredible effect on revenues.

We went through a period in the last year where we had a number of negative reviews that weren't necessarily fair but appeared on our listings.

And it immediately had a 20 to 30 percent impact on the individual unit sales.

But worse than that, it affected the relevance of those products in the Amazon search engine.

For Utah-based Brush Hero, there is a tipping point for negativity.

Even a few critical fake reviews could encourage genuine customers to pile on.

Once the review level drops below, say, a 3.5 out of five, we've noticed a vast increase of negative reviews that start occurring because it's just easier to pile on.

What that leads to is not just a loss of sales on Amazon and a loss of velocity on Amazon.

It also impacts sales all over the place because consumers are relatively savvy.

They're looking for review scores when they're buying off Amazon or Amazon.

So it decreases the efficacy of our advertising through Facebook, through other social media channels, through Google, whatever it is.

There is also a sense that the source of the reviews is difficult to address.

Amazon can do a lot to improve its review system.

I don't believe that they're doing a great job in tracking where reviews are coming from.

It seems to me that Amazon should have the statistical ability to identify those... those bad reviewers and vet them out of the system, and they haven't yet.

So where do the bad actors come from?

For Mr. Williams, the problem has one obvious source.

The sense is that it's coming from China.

When Amazon started to solicit Chinese sellers directly, it seemed to open the floodgates to a mountain of bad actors.

Despite his experiences, Mr. Williams is himself a devoted user of the platform.

Absolutely, it's changed the way I look at reviews.

I'm highly analytical about it.

I am an avid Amazon shopper.

I have to admit that even though I've had lots of problems on Amazon, it's a major sales channel for me.

And as a consumer, I purchase things nearly every day on Amazon.

The tangled web of online reviews is a challenge for Amazon, the companies that sell on it, and those of us buying.

But it's also just one part of a bigger story, the difficulty we now have trusting anything we read online, even as we spend more of our time in front of computers.