Correct me if I'm wrong, but from my understanding, any coin can set up a marketplace within Open Bazaar. I imagine all of the coins will do it. It's not like it's exclusive to XST. It's a decentralized Amazon.
I think it will come down to how well it is incorporated into the client - if it can be, and on merits of ease of use to end-users.
Also, you have to remember some coins may be deemed "safer" than others in terms of anonymity and people will trend toward using these safer coins in that type of market.
Coins that can do this very well (which I believe xst is capable of) will be ahead in the long run.
Those are good points.

Your wrong. The marketplace will be exclusive with xst. . Meaning it won't have or use bit coins public ledger or any other public ledger to operate. .. The whole system Wi operate anonymously
IMO the whole MArketplace idea should be a side project, it really needs a critical mass of users to even operate sucessfully and im fairly confident saying that the crypto market in general doesnt have the need (unless you get help from illegal sources ala silk road/btc i dont think the success can be replicated),, some coins already have this in place, we need something new and useful, not what everyone else is doin....
People like real development, new wallet functions ect.. something they can use, thats why CLOAK started off so well, in wallet exchange API was a really good idea, they made updates, wallet improvements, and and the dev was constantly active and delivering these are key to getting a strong market/commuinity build...