Setting up a shop:

1. Place a chest
2. Place a sign 1 block near the chest (for example, above the chest)
3. On the sign, write:

YOURNAME
Item Stacks
B Amount:Amount S
Item ID or Name

Example:

CoXeY
5
B 10:5 S
Eggs

This means that Coxey is selling Eggs in stacks of 5 for 10$, or that you can sell him 5 Eggs for 5$.

4. Fill the chest with the item your selling (for example, using the sign above would mean putting eggs in the chest).
5. Done!