Currently all you can buy are “Proto Repair-Bots” (tune your gear without visiting a tech in HQ) and extra Vault Tickets. Given that you usually leave the Vault with $US13 and a bunch of crap, there’s little reason to visit this Marketplace now.
A microtransaction basis for an MMO is, naturally, the way to take that thing to free-to-play. No one’s saying that will happen with DCUO, which released in January, but hey, it’s already there.
D.C. Universe Online Launches In-Game Market [Massively]