You're still assuming everything has a value that applies to everyone. I didn't say anything about fluctuating prices over time, but that value is entirely subjective. While a farmer will want one pencil and be willing to trade two cows for it, another may want 5 pencils for one cow.
If it was dollars, then in a barter economy, everything would be worth $1, and there would be a HUGE multiplier based on bartering skill (as much as 0.1x to 10x or whatever you want), current needs of the individual, etc. Raw "Bulk" items would probably need to be packed in certain amounts (so, i'm not going to trade you a single ear of corn, but maybe a sack, or bushel or whatever thing those things get packed into). Everything else (specially manufactured stuff) is fair game.
Again, if you were making the game you'd base the "AI" in some form of currency, but people who don't know about currency may have an entirely different heuristic in mind (pun?
) when they program it.
http://en.wikipedia.org/wiki/Nomad_(video_game)
In this game, everything is worth the same. One missile, one upgrade, one "unit" of goods. You meet an alien and always trade 1 for 1. They'll just say if they're interested or not.
Maybe too simplistic to be practical (only way to get new items is to somehow "produce" them from the wrecks of your dead enemies).