What it was
AlphaBay ran as a hidden service on Tor from 2014. Europol's account of the takedown says the market handled "a conservative estimation of USD 1 billion" in transactions since it started, and that payment was made "in Bitcoin and other cryptocurrencies". The Justice Department is more specific about which ones: bitcoin, monero and ether.
Both agencies give the same picture of its size. Europol records over 200,000 users and 40,000 vendors, more than 250,000 listings for illegal drugs and toxic chemicals, and over 100,000 listings for stolen and fraudulent identification documents, counterfeit goods, malware, hacking tools and firearms. For comparison, the Justice Department notes that Silk Road had roughly 14,000 listings when it was seized in 2013.
No legal entity has been established for the operation, so the structure is left undocumented rather than claimed as absent.
How it ended
The operation was led by the FBI and the DEA and was called Bayonet. It involved authorities in Thailand, the Netherlands, Lithuania, Canada, the United Kingdom and France, with Europol coordinating. Servers were seized in Canada, the Netherlands and Thailand, and the site went down on 20 July 2017. Dutch police had quietly taken over a second market, Hansa, a month earlier, so users leaving AlphaBay walked into a site the police were running.
On the record
Alexandre Cazes was charged in an indictment filed in the Eastern District of California on 1 June 2017, numbered 1:17-CR-00144-LJO, with conspiracy to engage in racketeering, conspiracy to distribute narcotics, six counts of distribution of narcotics, conspiracy to commit identity theft, four counts of unlawful transfer of false identification documents, conspiracy to commit access device fraud, trafficking in device making equipment and conspiracy to commit money laundering. Thai authorities arrested him on 5 July 2017 at the request of the United States. The Justice Department states that he died in custody in Thailand on 12 July 2017. The case never reached trial, and none of the charges was tested before a jury. As the same Justice Department release notes, "Charges contained in an indictment and/or complaint are merely allegations, and the defendant is presumed innocent unless and until proven guilty beyond a reasonable doubt in a court of law."
On 19 July 2017 the United States filed a civil forfeiture complaint against assets in Thailand, Cyprus, Liechtenstein and Antigua and Barbuda, and the FBI and DEA seized cryptocurrency the government identified as proceeds of the market.