Online Anonymous Marketplaces
By Kyle Soska and Nicolas Christin:
"The sale of contraband and illicit products on the Internet can probably be traced back to the origins of the Internet itself, with a number of forums and bulletin board systems where buyers and sellers could interact.
However, online markets have met with considerable developments in sophistication and scale, over the past six years or so, going from relatively confidential “classifieds”-type of listings such as on the Open Vendor Database, to large online anonymous marketplaces. Following the Silk Road blueprint, modern online anonymous markets run as Tor hidden services, which gives participants (marketplace operators and participants such as buyers and sellers) communication anonymity properties far superior to those available from alternative solutions (e.g., anonymous hosting); and use pseudonymous online currencies as payment systems (e.g., Bitcoin ) to make it possible to exchange money electronically without the immediate traceability that conventional payment systems (wire transfers, or credit card payments) provide.
The common point between all these marketplaces is that they actually are not themselves selling contraband. Instead, they are risk management platforms for participants in (mostly illegal) transactions. Risk is mitigated on several levels. First, by abolishing physical interactions between transacting parties, these marketplaces claim to reduce (or indeed, eliminate) the potential for physical violence during the transaction.
Second, by providing superior anonymity guarantees compared to the alternatives, online anonymous marketplaces shield – to some degree2 – transaction participants from law enforcement intervention.
Third, online anonymous marketplaces provide an escrow system to prevent financial risk. These systems are very similar in spirit to those developed by electronic 2Physical items still need to be delivered, which is a potential intervention point for law enforcement as shown in documented arrests.
Suppose Alice wants to purchase an item from Bob. Instead of directly paying Bob, she pays the marketplace operator, Oscar. Oscar then instructs Bob that he has received the payment, and that the item should be shipped. After Alice confirms receipt of the item, Oscar releases the money held in escrow to Bob. This allows the marketplace to adjudicate any dispute that could arise if Bob claims the item has been shipped, but Alice claims not to have received it. Some marketplaces claim to support Bitcoin’s recently standardized “multisig” feature which allows a transaction to be redeemed if, e.g., two out of three parties agree on its validity. For instance, Alice and Bob could agree the funds be transferred without Oscar’s explicit blessing, which prevents the escrow funds from being lost if the marketplace is seized or Oscar is incapacitated.
Fourth, and most importantly for our measurements, online anonymous marketplaces provide a feedback system to enforce quality control of the goods being sold. In marketplaces where feedback is mandatory, feedback is a good proxy to derive sales volumes . We will adopt a similar technique to estimate sales volumes.
At the time of this writing the Darknet Stats service  lists 28 active marketplaces. As illustrated in Fig. 1 for the Evolution and Agora marketplaces, marketplaces tend to have very similar interfaces, often loosely based on the original Silk Road user interface. Product categories (on the right in each screen capture) are typically self-selected by vendors. We discovered that categories are sometimes incorrectly chosen, which led us to build our own tools to properly categorize items. Feedback data (not shown in the figure) comes in various flavors.
Some marketplaces provide individual feedback per product and per transaction. This makes computation of sales volumes relatively easy as long as one can determine with good precision the time at which each piece of feedback was issued. Others provide feedback per vendor; if we can then link vendor feedback to specific items, we can again obtain a good estimate for sales volumes, but if not, we may not be able to derive any meaningful numbers. Last, in some marketplaces, feedback is either not mandatory, or only given as aggregates (e.g., “top 5% vendor”), which does not allow for detailed volume analysis." (https://www.usenix.org/system/files/conference/usenixsecurity15/sec15-paper-soska-updated.pdf)