I mean, that right there is why it would eventually work out just fine on PVE(-C) servers. Sure, I could keep grinding for the Final Breath of the Red Mother until I get one. Or I could buy it from the dude who was blessed by RNGeezus and got two in a row. 
Similarly, maybe I could cash in several of the legendaries that I don’t like and use that currency to buy myself some of the steel reinforcements that I lack for my new build and I don’t want to wait for them to be crafted.
Another way would be to implement an upkeep system where anything in the game could be converted into (different amounts of) “upkeep fuel”. That would establish an inherent value for each item in the game, which would work nicely with a trading system like this one.
In that scenario, the “upkeep fuel” and the “trading currency” (e.g. copper coins) could be the same thing.