Anyway, when she sold her stacked artifacts she only received gold for one item. She had to split the stacks in order to collect the money for each artifact. It seems that selling artifacts in stacks may not be correcting calculating the multiplier. So if you have any artifacts stacked up, be sure to split them into singles before you sell to the vendor. Considering that some artifacts can sell for 100 gold or more, you don't want to lose money this way.
It also can't hurt to post any problems you might have in the official support forums so Blizzard can look into the situation :)