Fraudsters are earning an estimated $137bn a year by selling counterfeit goods, according to brand protection firm MarkMonitor.
Highly sophisticated criminals are using the latest search engine optimisation techniques and investing millions in abusing online marketing tools such as Google's AdWords to sell fake goods, the company claims.
A 10-day survey by the firm of three top ski-wear brands in December found more than 46,500 listings on auction sites and 800 listings on exchange sites suspected of selling counterfeit goods.
"If discounts on branded goods are 50% and higher, it is highly likely that the goods on offer are counterfeit," said Charlie Abrahams, vice-president and general manager for MarkMonitor.
Anyone buying hugely discounted branded goods online runs the risk of getting fakes and putting their identity at risk by giving bank details to the sellers of these goods, he said.
The biggest losers are brand owners and legitimate retailers, who typically get overwhelmed by the sheer volume of websites selling counterfeit goods, according to Abrahams.
There are at least eight ways brand owners can tackle the problem, but they need to get involved because they can best identify counterfeits, he said.
Deterrents range in cost from low-effort, high-volume methods such as blacklisting illegitimate websites and automated cease and desist letters, to high-effort, low-volume methods such as reclaiming hijacked domain names.
The lower-cost methods can reduce the volume to more manageable levels to enable brand owners to spend time and money taking down only the most persistent and harmful infringers, said Abrahams.
Like legitimate retailers, counterfeit sellers measure their return on investment. If brand owners make it difficult for them to operate, they will usually move on to follow the line of least resistance, he said.