For sure could get out of hand. I think with a progression system it will all need refining with the economy anyways, especially if they plan to go very deep or wide with it.
As for requesting items, I feel like it should be enough costs to know its not the most efficient way, but if you are just having bad luck with prospecting or clay or whatever else? Could be a good way to get resources you need and cant find.
Could even open up some other types of play, for instance I thought about me and my friends next world being a huge ocean with islands, so resources might be more scarce, having traders be able to grab specific items for you from far away could be interesting in that setup.
The idea of having randomness to it is a good idea s well I feel. Heck that could even tie into a skill or relation based system. Could also be interesting to if you ask them for a rare enough item they be gone from their spot a day or two, or even die? Who knows, there are a lot of nifty things you could tie into the system I feel.