Sheep Marketplace, a darknet shopping site, has shut down following a catastrophic theft of 5,400 bitcoins, currently valued at over £3.5m.

The site uses technologies such as Tor and Bitcoin to enable users to purchase illegal goods online, and is one of a spate of successors to the Silk Road site which was shut down in October. For a few days, visitors to Sheep received a notice blaming the theft for the closure, but now its entire online presence has been removed.

Sheep's administrators claim that the theft was carried out by a dealer with the username EBOOK101. "This vendor found a bug in our system and stole 5,400 BTC – your money, our provisions, all was stolen," they wrote in the online message.

But far more than 5,400 BTC remained in Sheep's Bitcoin wallets after the closure, and disgruntled users are accusing the site's owners of stealing almost 100,000 BTC from their accounts.

Fears had already been raised over the security of the site. In October, members of the Sheep Marketplace subforum on social news site Reddit discovered glaring holes linking Sheep and an apparently unofficial normal website, sheepmarketplace.com. The latter existed as an online signpost to the former, but because it was an unprotected website, it let users discover the location of the black marketplace's owner. "sheepmarketplace.com's owner is the same as Sheep Marketplace; he is living in Czech Republic; he sucks at security," concluded one Redditor.

Following the closure of Sheep, a second black marketplace, Black Market Reloaded (BMR), also announced that it would be closing down. BMR is currently the largest of the darknet marketplaces by volume of goods on sale, but its admins are worried by that fact.

"Tor can't support any site to be too big," wrote backopy, BMR's founder, on its official forums. "Without competition the wisest thing to do is to shutdown the market, doing it in a timely and orderly manner."

• Black Market Reloaded was itself closed briefly in October, following a leak of its code