On the successor to the notorious online drugs emporium Silk Road, a gram of uncut Peruvian cocaine could be bought for $100, and a gram of ketamine for $37.

Yet the best-selling item on the site, which could only be accessed through the anonymous "darknet", was something a little more banal – Tesco Clubcard vouchers.

Before the site was taken down, screengrabs show that Clubcard vouchers worth £100 were being sold for £43 by one vendor.

The vendors were given high satisfaction ratings by their scores of happy customers, perhaps taking a break from searching for automatic weapons or narcotics.

Fraudsters copy and print the vouchers using computer technology, with the Institute of Sales Promotion estimating that voucher fraud could be costing stores up to £300,000 a year.

One customer expressed concern that Tesco would become suspicious if they noticed someone only paying for groceries with £100 and £50 vouchers. Another offers the reassurance: "I tried it, I bought £100 from Revivalry (vendor on SR) around Christmas time, it worked in store not sure about online never bought online vouchers."

The site was recently closed, after customers were robbed of £2 million worth of the virtual currency bitcoin.

Tesco said it was aware of the illegal sale of its vouchers online, and was working with police.

"We have referred the matter to the police and will assist them with their inquiries," said a spokesman.

The original Silk Road website was closed down by authorities in America in 2013, and its alleged founder, Ross Ulbricht, aka Dread Pirate Roberts, faces years in jail if found guilty at trial later this year.

However, several replacement sites soon sprang up on the Tor network, which is accessed using special encrypting technology to ensure anonymity, with experts warning that it may be impossible for authorities to completely eradicate the sale of illicit goods on the network.

"The amount of money going through these websites is unbelievable," Gareth Balmer of the treatment charity Addaction told the Guardian.

He suggested the internet could transform the way drugs were distributed in the same way that it had transformed the way music and film were sold.

"Ten years ago, downloading was a niche market. But now it's much easier and more straightforward. It is possible that we are now on the verge of that happening with the dark net markets," he said.