British Museum gems for sale on eBay - how a theft was exposed

In 2020, Danish antiquities dealer Dr Ittai Gradel began to suspect an eBay seller he had been buying from was a thief who was stealing from the British Museum.
More than two years later, the museum would announce that thousands of objects were missing, stolen or damaged from its collection. It had finally believed Dr Gradel - but why had it taken so long for it to do so?