I know I'm late to the party by a while but I thought I'd drop in my two cents.
If there was to be an economy system, and I'm not against bartering mind you, it would need to be something quite substantial and dynamic.
Let's say for instance that there was a small town near the woods, home to lumberjacks and hunters and they had a trade, or were owned by, a larger town, keep, city or military compound, whatever it may be. They delivered supplies there so if you went to either one you could by furs and logs, but buying furs and logs straight from the source might be cheaper, or it might be even more cheaper if you bought it from the larger town if they had an overstock, maybe the smaller town gave them too much and they didn't know what to do with it. Then you go to another town a while away and they'll buy your pelts, furs and other items for double the price of what you bought it for as they don't have a smaller town that supplies them and have to make do with buying and selling.
Now let's shake things up a bit. Let's say that, one day, the caravan that normally passes by your base every week carrying valuable goods (muskets, bows, other weapons or tools, whatever it may be), but this time they don't. You go to the town where you can normally buy some musket/rifle bullets for one coin a bullet or fifty coins a rifle, for example. Unfortunately, the town or factory that supplied them has been attacked by bandits, raiders, monsters, jellybean men, that sort of thing. Now they can't get their supplies until they take that town back and either save the population or repopulate the place with settlers. Those bullets that were dirt cheap before? Ten to fifteen coins each. The rifles? 150 coins please, we can't make them ourselves and only have thirty left.
But I like bartering as well. See that survival compound all the way up in the mountains with generators, solar panels and guys armed with scoped rifles? They don't care about money, they want nothing to do with all the other people out there. All they're concerned about is their crops, their hunting, their safety and the small groups of people that wander to and from the survival compound trading valuable scrap stuff for food. What do they trade? Well, like some other people above me suggested with fish and stuff, they want solar panels. What do you get in return? Food, water, advanced technology (solar panels and the like, obviously no spaceship stuff as these guys can barely keep their clothes stitched together), that sort of thing. Stuff for stuff basically, with stuff that's more valuable getting you more stuff of a lesser value, like a bullet for five apples or something.
This is all from a purely single-player perspective, as I don't play multiplayer at all (yet). Plus, that's all advanced stuff and I don't know if even a big dev team could make something that dynamic, but that's the way I'd see an economy system working, plus bartering as well. I suppose you could also take this system and make it a multiplayer type of thing, but it would need to be something like EVE: Online to be a true economy system with players running guilds, kingdoms, whatever.
But yeah, that's my idea. Not quite sure if that's what you wanted but it's the first idea that sprang to mind after reading your post and I had to write it down before I forgot it.