If anything double deposit will limit the amount of buyers/sellers visit the marketplace.
Wait a second... you do understand that in that case (employee) it would be absurd to require a deposit for the employee? In that case they are the same as the vendor actually... providing a service. If they are unknown and providing a service for a very high rate without proof of competency... then a DD escrow contract would make sense... on the other hand if they are asking a reasonable rate and willing to accept my offer (via smart contract I would draft) then I simply agree to pay smaller amounts over time as deadlines are met rather than a single contract for the entire amount upfront.
... Exactly the same as I would do for someone that was physically hired as an employee on-site, with nothing but a resume (i.e. without a reputation I could simply verify with colleagues for example).
Nothing is required on either side of a smart contract... what makes it 'smart' however is that stipulations can be integrated on one or both sides in cases where other means of assuring 'fair play' from both parties is not available. There's nothing to say you can't sell something with no deposit required at all... also nothing saying that they (or you) can't simply violate that contract with little repercussion (other than a reputation ding)... just like any other online marketplace.
The difference is that decentralization requires a different approach to difficult situations... because there's no running to the police/moderators/government/etc. and crying "foul".