You must first have a Vault where all the payments from any purchases will go into. You can ask a Moderator to help you set one up to get started.
Place the items you want to sell in the chest, do this so the sign can automatically detect the items you want to sell. Only one-type of item per chest!
Set up your sign to start selling your items! Use the follow of the image below.
[X] = how much buyers will pay
[Y] = how much you are willing to pay if they want to sell it back
Put ? (question mark) on the last line, it will automatically detect the item you are selling in that chest
You may also only specify B [x] only or S [y] only for only buy or only sell