probably best if you have someone show you in-game, but basically you place a chest, put what you want to sell in it (say 8 iron ingots), then place a sign in this format:
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text
more text
$priceyouwanttosellfor($100 for example)
[sell] (or [buy] if you want to buy things from players)
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next, get a piece of redstone and rightclick (or maybe left, idk been a while) on the chest once, then rightclick (or left) on the sign. you should get a message the shop has been created
couple notes. you can only have 20 signs. when making the shop ONLY put in the chest the amount you want to dish out, add the rest after linking the sign. signs can share a chest if you need to save space, but make sure to take everything out and only have the items for the new sign in the chest when you click it.
think that is everything, I'm sure someone will point out what I missed.