The Hydra Market's bitcoin privacy mixer complicated the investigation, police said.
The cybercrime office of the Frankfurt prosecutor's office and federal criminal police confiscated 543 BTC as they "secured" the site's servers, according to a federal police statement on Tuesday.
Federal police found 17 million customers and 19,000 seller accounts. Hydra Market probably had the highest turnover among illegal marketplaces in the world, according to the statement. In 2020, the marketplace had €1.23 billion ($1.35 billion) in revenue, the police said.
The Russian-language marketplace also had a built-in bitcoin privacy mixer, which complicated tracing the transactions, said the police statement.
Hydra Market had been operating since 2015 and was accessible via the Tor browser. The site has been taken off the internet.
The market was mainly used for narcotics, and served Russia, Ukraine, Belarus, Kazakhstan, Azerbaijan, Armenia, Kyrgyzstan, Uzbekistan, Tajikistan and Moldova, according to blockchain forensics firm Ciphertrace.
The investigation started in August 2021 and also involved U.S. authorities, the police said.