Place a Chest, place a Sign on a block behind the chest. (Or in front of the chest on the ground, Unsure of this way.)
Sign Layout
Line 1 Your Name
Line 2 The Amount You're selling.
Line 3 B (Amount)
Line 4 The ID of the Item.
EX.
Dogspaw
1
B 3
265
Which will then turn into
DogsPaw
1
B 3
Iron Ingot
In your Chat it should say that your shop has been created.