An alternative method is to make "recipes", items that are dummies, cost zero, but have all the appearance of the mixed item. This is what you'd need:
Orb of Frost (500 gp, 250 sell value, slowing effect)
Orb of Fire (500 gp, 250 sell value, splash effect)
or w/e
Orb of Elements (requires frost and fire orbs, 500 sell value, slowing/splash effect)
Orb of Elements Recipe (same tooltips and description as element orb, only 0 cost and no abilities)
So when you buy the recipe, it then checks to see if you have both of the frost and fire orbs. If you do, the recipe and both orbs should be removed, and the elemental orb put in the hero's inventory. If you don't, the recipe vanishes.
It's just another method of doing it, instead of having a set of triggers for aquiring each part of a recipe. That's a bit complex.
However, if you need to ask what the specific triggers are to do any of this, you shouldn't be asking how in the first place. Learn your triggers.