Wouldn't it be a great honeypot for one big bust? Delay the launch as long as possible due to "load issues" to bring in as many sellers as possible and then take them all down after their first transaction.
How are you going to find a seller? They use Tor, don't need to enter their details anywhere; they just send packages to addresses they receive in encrypted form.
Most sellers are too lazy/cheap to properly set-up their fulfillment operation with good enough security. If you read accounts of the seller busts from Silk Road 1.0, the authorities can trace where your packages have been sent from and where you bought the postage from. If you don't operate as if the police are actively using this information to track you down, they will be able to.
Sellers don't reveal their personal details to the site - maybe having a honeypot site would make it easier to track them down, but it wouldn't be simple.
