Shop restocks on the first of every month. Cheaper items like orc, chronos and glad restock every month and usually sell out right away.
Depending on the amount of items that were bought from the shop in a month, those that are out of stock or low are restocked. Sometimes certain items are restocked, other times they aren't.
For certain items it's cheaper to buy from shop, as people will try to control the market/forum prices by buying all of them and selling them really expensive.
Normally items in the forums are the cheapest way to get them.