The number indicates the minute it restocks. For example for the shops that restock at 0/5 every time the minutes ends in 0 or 5 is when it will restock - as in 1:00, 1:05, 1:10, 1:15, 1:20, 1:25 etc. It is the same for the rest 1/6 would be 1:01, 1:06 and so forth.