This runs on bitmessage over Tor. Tor is just another layer to make it more difficult for merchant to recognize your location.
Think of it this way:
1) Bitmessage ensures that FBI has to catch merchants or buyers individually instead of catching them in one single server like SR.
2) Tor makes it harder for them to locate a party by tracing where message came from.
Tor is not perfect, but Bitmessage is not better, but worse than Tor in terms of tracing the origin of the message (because BM does not use onion routing with layers of encryption).
How cam bitmessage possibly be worse than Tor? All that bitmessage indicates is that you're taking part in the network, not what you're actually saying or who you're talking to.
When Alice and Bob communicate over Bitmessage, Eve can only see garbage being propagated through the network.
But when Alice sends a message to Bob (who is an undercover TLA agent), Bob could monitor traffic and statistically figure that certain IP address sends messages addressed to him earlier than other nodes. So he goes and busts the owner of that IP address.
Onion routing attempts to make this discovery harder for Bob because Bob will always receive messages from different IPs that supposedly do not belong to him. They have little value to him because they themselves are connected to relaying nodes, not to initial senders. Bob would have to bust too many IPs before finding the sender, which could be too expensive.
Onion routing begins to fail when Bob has a lot of his own nodes in the Tor network, so for a large enough number of messages he can trace route back to the sender's physical address.
To achieve real anonymity when chatting with strangers (e.g. blackmarket merchants), one needs to use a combination of these factors:
1) Bitmessage or alike to avoid evesdropping.
2) Tor to make it harder for recipient to find location of the sender.
3) Low-latency network to make statistical analysis less efficient. Every relaying node (both Tor and Bitmessage) should delay broadcasting messages randomly.
4) Infrequent communication, so it takes time for recipient to gather data. (This is a variant of #3)
5) Change physical location frequently, randomly and rarely reuse them. E.g. connect from various free wi-fi points in cafes, parks, shops, Apple Stores etc.
6) Never reuse identity between people you communicate with. Merchants must have separate Bitmessage and Bitcoin address per invoice (once item is sold, post another item with different identity). Buyers must use different Bitmessage and Bitcoin address for each purchase. This way amount of information available to an adversary will be strictly limited to just one deal. And that deal will be limited to one unique location and a few exchanged messages that hopefully won't be enough to locate the person. And even if that happens, person couldn't be charged with more than one sin.
If you communicate with people you trust (friends, family members), you only need #1 and that would be enough.
Think of it this way:
1) Bitmessage ensures that FBI has to catch merchants or buyers individually instead of catching them in one single server like SR. 2) Tor makes it harder for them to locate a party by tracing where message came from.
Tor is not perfect, but Bitmessage is not better, but worse than Tor in terms of tracing the origin of the message (because BM does not use onion routing with layers of encryption).