anyone can make a shop sign
set up a sign on a chest in this order
[ign]
[amount selling/buying per click]
[b price] or [s price] b is for the customer buying, and s for the customer selling
[item name] sometimes you have to do /iinfo with the item in hand to get the actual item name
when done, it should look something like what is in the attached screenshot
if you still are having trouble, I'm sure someone will be able to help you in game