I would tell them I purchased the items back home should they start questioning. They may say that I don't have the prove that such items were indeed purchased at home but at the same token, they don't have the prove to say that I purchased it overseas.

Look...just relax. Tourists travel out of the country all the time with their cameras and very seldomly do they ever be questioned upon return home at customs regarding the location of the purchase. People would have stopped bringing cameras on their vacation long time ago if they have such paranoia that they'll have to pay duty everytime they bring their cameras overseas.