Secondary ticketing refers to the practice of reselling tickets for an event, such as a rock concert or a football match.

The law regarding secondary sale of football tickets is clear: it is illegal unless the club has given express authorisation, such as Manchester City’s partnership with Viagogo. However, it is still perfectly legal to resell tickets for other events such as gigs and concerts, as long as the sale abides by consumer protection laws.

In the past, the secondary market was dominated by street-based touts operating outside venues, buying tickets cheaply from people who had spares and selling them on to last-minute buyers. While street touts still exist, the advent of the internet has transformed the industry.

These days the secondary market is dominated by four major players – StubHub, Viagogo, Seatwave and GetMeIn – which provide online platforms for people to sell on tickets, often at inflated prices, for artists such as Adele and Metallica.

While many of the people using these sites are genuine fans who cannot attend an event and want to recoup their money, in recent years the practice has become increasingly dominated by a relatively small group of “armchair” touts.

These traders use a variety of methods, some exposed in previous Guardian reporting, to harvest hundreds of tickets before genuine fans can get hold of them. They can list them in bulk on the secondary platforms and cash in from the comfort of their homes.

For the most in-demand events the tickets are often sold at many times their face value. Touts have built multimillion-pound businesses, some based offshore, on the back of this.

Over the past few years there have been many examples of major secondary ticketing companies breaching consumer laws. For instance, some have not disclosed to buyers that the terms and conditions of their ticket could result in them being turned away at the door, if a promoter or venue has a strict anti-touting policy.

Other firms have sold tickets that do not carry any information about where the seat will be.

The government recently tightened up the law, implementing legal changes that require sellers to furnish buyers with as much information as possible about the tickets and that sellers disclose any restrictions.

Meanwhile, the Competition and Markets Authority is conducting an investigation into whether the secondary market is fair and has said it is considering legal action against Viagogo. It raided the London offices of the Switzerland-based firm and competitor StubHub last year.

National Trading Standards, the government agency that tackles rogue traders, is looking into whether the actions of touts are lawful and made four arrests last year. However, secondary ticketing platforms are still dominated by professional touts listing hundreds of tickets at once and making tidy profits. The websites, of course, take a handsome cut.