Last Tuesday, the darknet market known as AlphaBay vanished without a trace. With more than 100,000 users and an estimated $800,000 in daily transactions, it was the largest market in the underground web. AlphaBay administrators initially appeared on Reddit to assure users that the site was undergoing routine maintenance, but with each passing day, the story seemed less and less plausible. Now that more than a week has elapsed, it looks like AlphaBay may have pulled the ultimate exit scam.

Darknet markets are hidden websites accessible through browsers such as Tor, which route encrypted communications through multiple machines to mask the user’s location. Each site hosts a unique selection of vendors peddling illicit goods such as drugs, weapons and stolen data. Darknet participants are not exactly beacons of trust, so sites like AlphaBay offer an escrow service to hold bitcoin payments until a buyer confirms receipt of a delivery. Market operators typically charge a 5-10 percent fee per transaction.

The escrow service creates a separate risk to the vendors: The larger the marketplace, the greater the amount of bitcoin in custody at any given time. A sizable market might be holding several million dollars’ worth of bitcoin in escrow, and vendors have to trust the site administrators not to shut it all down and run off with the deposits — a move known as an exit scam. Two years ago, a market called Evolution exited with an estimated $12 million worth of bitcoin.

It’s not a question of whether exit scams will occur; rather, it’s a question of when. The anonymous nature of these sites means that participants have no recourse in the event of wrongdoing. Market operators have to calculate…