In barter it is a matter of offer and acceptance. You see something you need you offer something you got. If the other person likes the deal it works.
Read Alas Babylon. It gives you a couple of scenes of trading.
What you have to trade is only worth what someone else is willing to offer. In a true barter system there are no fixed prices.
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When in danger or in doubt
run in circles scream and shout
RAH
And always remember TANSTAAFL