On the one hand, allowing trading between players (for credits, mats, cargo, whatever) would certainly open up more kinds of gameplay.
On the other hand, it also opens massive cans of worms - gold sellers for one thing, sure, but there are also things like economic balance (currently it doesn't affect me at all if the next player really does grind Void Opals all day, if players can trade things it will, and then I might have to do that as well, and then since everyone is insanely rich the prices for everything even sold by NPCs have to be increased to make money meaningful, and frankly there's a reason that EvE has an economist on staff to sort this stuff out...).
So that's a judgement call that's ultimately up to FD to make. A player marketplace would be great if done well, an utter disaster if done badly, and not having one at all sidesteps the problem completely. I respect FD's right to do that.