Either read this reply or check the link Danniel1998 posted

It is very simple really to set up a chest shop it is just not that easy to explain it but the way you do it is
Sign Line 1: You write your name here if you want but you do not have to cause the shop will auto write your name when you done the next lines and clicked done on the sign
Sign Line 2: This is the Amount of items you will sell for the price that you put in Line 3
Sign Line 3: Here you can type in for example B 2:S 2, B is for Buy that means people can buy the item for the price you set (Dirt for 1 noob = B 1 and Dirt on the 4th line) and S is for selling that means they also can sell the item you write on the 4th line for the amount you choice example if you want to buy a disc 11 you can write "b 0:S 100" on the sign that means you buy the disc for 100nbz.
Sign Line 4: This is where you write what you want to sell for the price at line 3 and the amount of it on line 2.
Small set up that you can use if you sell dirt for 4nbz each and buy it for 1nbz
(Name)
1
B 4:S 1
Dirt
Hope this helped you