I also don't get this how do we as the customers know if its legit or not like what? people could literally just sell 50m/ 100m at a time claiming its legit even though they bought billions from someone. Maybe someone here knows.

You don't,there is no way to know,until blizzard act on this matter i assume all the gold sold on any website or jsp to be duped.
Doesn't matter what the seller says,there is no way to prove that his gold is not duped.

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