Alright, in the Object Editor you create a new item, that does absolutely nothing (a dummy item) and add it to the shop's "Items Sold" list.
When a hero buys that item, the item he bought is removed from his inventory and the hero gains a random item of level 1.
For the player to know item of what level he's going to buy, name the item "Random Level 1 Item"
Then you add in Object Editor "Random Level 2 Item" and so on. When hero buys "Random Level 2 Item" that item is removed from the game, and the hero acquires an absolutely random item of level 2 VIA TRIGGERS.
Will that go ok?
And here's another idea:
You make a shop with empty "Items Sold" list. In some moment via triggers you add 11 random to be sold in this shop. When you want to add different random items for sale in the shop, you simply replace the existing shop with a new one, that has initially empty "Items Sold" field, as set in Object Editor. Then you simply add random items again to a new, clear, empty shop.