Breach of copyright is a civil and criminal offence. Take a look at certain market stalls and Boat Jumbles and you will see items removed by the police and trading standards officers. In Ireland the provo's and CIRA trade in computer software, games and dvd's. Lifted by Garda and RUC so why not the Met?
For example, in all the leading yachting journals I frequently see in the small ads section photos of my own boat used to sell boats of the same class. I am sure that the photographer was not consulted each time. But, unfortunately for the photographer, the advertisers and the magazines are not committing a criminal offence.
Similarly, many postings on these bulletin boards could potentially be in breach of copyright eg. where advice has been lifted from elsewhere. There's no immediate need to lock up the posters, though.
Incidentally, there is no such thing as a "civil offence". Rather, breach of copyright gives you a right to a civil action for damages.
It's not surprising that the trading standards officer and police were searching the market stalls. Selling items with a false description (ie. not originals) would be a breach of trading standards. The police were probably looking for stolen items or may also have been interested in breaches of trading standards.