The site, known as DarkMarket, was shut down today, German prosecutors said.
Drugs, forged money, stolen or forged credit cards, anonymous mobile phone SIM cards and malware were among the things offered for sale there, prosecutors allege.
Prosecutors said a judge has ordered him held in custody pending possible formal charges, and he hasn’t given any information to investigators.
Australian police assisted German investigators in their months-long probe, alongside US, British, Danish, Swiss, Ukrainian and Moldovan law enforcement.
An Australian Federal Police (AFP) spokesperson said it was “working closer than ever” with European authorities to tackle dark web and and organised crime threats impacting Australia.
“Our message is clear,” the AFP spokesperson said.
“If Australians are considering using a dark net marketplace to conceal criminal activity, your anonymity is not guaranteed and you are not outside the reach of law enforcement.”
The takedown of DarkMarket was significant, the spokesperson said, “with impacts around the world on thousands of Dark Market vendors and users”.
DarkMarket had nearly 500,000 users and more than 2400 vendors, German prosecutors said.
They added that it processed more than 320,000 transactions, and Bitcoin and Monero cryptocurrency to the value of more than 140 million euros ($219.8 million) were exchanged.
More than 20 servers in Moldova and Ukraine were seized, German prosecutors said. They hope to find information on those servers about other participants in the marketplace.
Prosecutors said the move against DarkMarket originated in an investigation of a data processing centre installed in a former NATO bunker in southwestern Germany that hosted sites dealing in drugs and other illegal activities. It was shut down in 2019.
That centre hosted DarkMarket at one point.