Seems to me that any dark service that doesn't do it that way probably shouldn't be trusted.
>That system, would require at least two out of three parties in a transaction—the buyer, the seller, and Evolution’s administrators—to sign off on a deal. But due to its complexity, buyers rarely used the feature.
Edit: apparently this does not use bitcoins version of it, but their own that requires them to hold the funds in escrow. Which seems pretty stupid.
Is the arbitrator not a third party?
The transaction simply moves funds between buyer and seller. If buyer and seller both sign the transaction, it completes without the arbitrator's involvement. If buyer and seller dispute, then the one who hopes to complete the transaction signs it and contacts the arbitrator, who will either sign, or not sign, but either way never gets possession of the funds.