Some1 help me about shop plz!

I dunno how to explain this. But i notice that in the blizzard's campaign i saw a building (shop), but when i move there, it sells nth and can't buy anything, it is just acting like a normal building. But after i completed the quest, the building began to change to shop, than i can buy items frm there.

But when i try to do so in my map, i can't get this right.
I hope some1 can tell me. But plz dun "ask me to open the camapign with whatever programme"

Does this need triggers?
I think replacing the building with another is not a good idea. Cos it will slightly move away frm its main location.

Hope some1 help me. Thx in advance!

With triggers it could be possible to add select hero ability.
Then the shop will select you and you can buy items.
I don't know if it's possible to add that exact ability, but it's not a hero ability,
so it should be fine.

no, i'm pritty sure theres a trigger that allows you to add items to a shop or allow certain items