The way I would do it would be to make items which appear with the same tooltips as the abilities, then give them to a store. When purchased, give the ability to the hero if they haven't learnt it already, if they have then upgrade it. I think the trigger would be something like this:
Event--
Any Unit Aquires an Item
Condition--
Purchasing Unit equal to [Players Hero] (You would make an OR statement for multiple variables for heroes, or ignore this if you want any unit to be able to purchase it)
Action--
IF Item-Type of [Item Being Manipulated] equal to [Item Name]
THEN Remove [Item Being Manipulated]
--- IF Level of [Ability Name] Equal to [0]
---THEN Add Ability [Ability Name] to [Unit Manipulating Item]
---ELSE Increase level of [Ability Name] for [Unit Manipulating Item]
ELSE
Do Nothing
Repeat that for each ability then end with
Remove [Item Being Manipulated]
I think that's at least close.
And I believe MurderMode is right, this should have been posted in the Triggering section.