From the article:
Some Viking swords were among the best ever made, still fearsome weapons after a millennium. The legendary swords found at Viking sites across northern Europe bear the maker's name, Ulfberht, in raised letters at the hilt end. Puzzlingly, so do the worst ones, found in fragments on battle sites or in graves.
The Vikings would have found it impossible to tell the difference when they bought a newly forged sword: both would have looked identical, and had razor sharp blades. The difference would have only emerged in use, often fatally.