How effectively do big Internet auction companies and business-to-business portals police their sites? That’s been a hotly disputed topic for some time. In recent years companies have sought to improve their processes, but some have still been dogged by complaints.

On Monday the biggest B2B online commerce site in China, and one of the biggest in the world — — admitted that it has been riddled with fraud for years. And two top executives announced they were taking responsibility for failing to stop it by resigning immediately.