Call Samsung Careline and quote them the phone imei number. They can tell you if the phone is SME or AP or non-existant (clone)

Buy from Samsung origin retailer shop (or telco shop) is the safer way, or bring someone really known about phone and better with one having real S III unit. Also always remember don't greedy on cheaper price, nothing in world is FOC or far far cheaper than suppose price.

Some tips:
The link you show me, I can immediate tell it was clone as they is two SIM card slot, one regular, one microSIM, no manufacture will ever do such, also bear in mind no manufacture will ever do dual SIM also for high end phone. (Only those clone will ever do that).

Beside this, search more internet review and photos, especially GSMArena and PhoneArena, then you can easier rectified it was origin or clone. From the OS screen, the pull down notification is the most obvious to tell it was not origin. The status bar clock & battery icon also in wrong ICS blue color, Samsung still using green battery icon and the S III clock was in white grey.

The setting menu in origin Android look tell this was clone as well.

Also the Accuweather was the old design (from S II) also immediate tell this was not the origin.