There was actually a nice little (read: long) thread on economics issues. Money is nice, but if it doesn't do anything, and isn't programmed as a hard currency, then few would bother to use it. There's also problems with inflation and deflation. I think regional resources and high rates of item decay (like when stuff goes boom in Eve) would do wonders to help develop a real economy where people and certain, small groups couldn't be self-sufficient.
not all tribes will have access to the junk piles for scavenging things so in a way that makes it so that some tribes or solo players will have to trade with other tribes