So, here's how I envision it working.
You download a client, which is similar to SelfSovereignty's MetaSilk. In fact, MetaSilk might be a good starting point for such a client. There could be many different clients created by different people, like the variouis bitcoin clients, and you could choose who you trust.
The client launches a Tor instance and connects to other nodes in the network, and downloads/updates the database. Obviously, some nodes will have to run as hidden services, so there would be an option to create a hidden service for your client to let other clients reach you and download the database. In practice, a small percentage of nodes would run as hidden services, while most people would run their clients in non-hidden service mode, but once they are connected to other nodes, they can fully participate in the market.
The client manages your keys. The first time you run it, it generates a longterm identity key, which you will want to back up so you can maintain your buyer/seller history. It also generates one-time transaction keys and calculates stats when you are given other people's identity keys.
The client also manages your funds. It's like a bitcoin-qt client in that way. It maintains a wallet.dat, or perhaps multiple indepedent wallets for added privacy. You load coins into your "buyer account" by sending them to the bitcoin addresses in your market client.
It has an interface for searching and viewing listings. When you are ready to buy, you click a button to submit the transaction, which is signed with a one-time key signed by your identity key.
You can also submit listings at zero cost.
The most important part is that there's an option to include an escrow agent. You enter the agent's identity key (the Silk Road Escrow Service identity key would be well known, for example), and your client contacts his client (which is a hidden service) to initiate the transaction. You would only have to do this once, and your client would store the escrow key for all future transactions. The escrow client could simply be a marketd, running on a server, with scripts that automate the escrow process until human interaction is needed during resolution.
The escrow client initiates a transaction with encrypted data that the operator can't see. The only thing he sees is the BTC value, from which he will receive a fee. Each escrow agent can set his own fees. You agree to the fee when you elect to use the escrow agent's key and send him a notice to initiate the transaction.
The escrowed transaction has a time limit. The buyer and seller must sign it again within a certain amount of time, or the contents of the transaction are revealed to the escrow agent. This is effectively the same as finalizing.
** I'm not sure exactly how this time-based decryption system would work. This is where I need your help kmf. **
If the transaction is disputed, the escrow agent resolves it and signs the transaction to either return the funds to the buyer, send them to the seller, or some kind of split.
If the transaction completes successfully, as presumably > 90% of transactions would, both parties sign it and the funds are sent to the seller.
There are other things that could be built on top of this sytem. For lazy people, those who don't want to run desktop clients, or those who don't care about their security, there could be hidden service web sites where they could create accounts, and which manage all of this stuff for them. In fact, SR could be an interface for this. It's analogous to people choosing between hosting their coins on an ewallet or with their own bitcoin-qt client, even though everyone knows it's safer to run your own bitcoin-qt client. If people choose to use a third party service, that's no worse for their privacy than the current situation. In fact, it's exactly the same.
The great thing is that the user has choice in every step of the process. You can choose to use a third party market service or not. You can choose to use an escrow service or not. You can choose to reveal more info or not. There is no default transaction history or buyer stats that someone must know in order for you to participate.
Now, who wants to build it?