The mantra that gamers fall back on when reading game rules is do exactly what the rules say in the order written.
In this situation if all the items are supposed to be artifacts I would expect the card to be written:
"2 common artifacts, 2 epic artifacts."
Artifacts PLEURAL, and directly referring to both the common and epic artifacts.
In separating the "common" from the "epic" with the comma and having the descriptor "artifact" only after the "epic" I would expect the shop to be stocked with 2 common items of any kind and 2 epic "artifacts".
+1 This one. I believe I read it somewhere that this interpretation is the correct one. I just don't recall where.