replace items in shop

is there any way, be it with triggers or normal shop function, to replace an item in a shop when it is bought? i am using a rank based system and every time a rank of an item is bought it needs to be replaced in the shop by a higher rank.

any thoughts?

Yup. First, remove all the items from the shop's items sold field. Then, in your map initialization trigger, use the "Neutral Marketplace" category actions to add the the shop's initial inventory to the shop. Then, make a trigger that when a unit acquires an item of a given type, use the Neutral Marketplace actions to remove that item from the shop and add the item of the higher class to the shop.