A grey and white piece of a cameo gemstone featuring Priapus - the Greek god of fertility - was posted by a user called "sultan1966" for just £40.
The listing was taken down after a few hours, unsold.
Priapus is one of the more memorable gods - often depicted with his oversized genitals on show. The seller may have hoped Priapus' brief appearance online had gone unnoticed - but alas not.
It was spotted by Dr Gradel, who says he was born with a photographic memory.
He says the unusual skill has helped him identify rare finds. In this case, it would also help him uncover the identity of a suspected thief.
Dr Gradel had been buying gemstones from sultan1966 on eBay for almost two years. The seller had told him his name was Paul Higgins and that he had inherited the gems from a grandfather. They were being sold at bargain prices but Dr Gradel could tell many were very valuable.
This time, he knew he had seen the Priapus cameo before. He was sure it featured in an old gems catalogue he owned from one of the world’s most famous institutions, the British Museum.
“There was no doubt it was the same object and I was confused,” he says.