The British Museum announced last year that ancient gems, jewellery and other items from its collection, were missing, stolen or damaged.
One buyer, based in New Orleans, told the BBC an FBI agent had emailed him asking for information about two pieces he had bought on eBay.
The FBI agent said they were assisting the Metropolitan Police with investigating missing or stolen items from the museum.
The New Orleans buyer, Tonio Birbiglia, told the BBC he had bought two items from sultan1966.
One was an amethyst gem depicting a Cupid - the Roman god of love - riding a dolphin, which Mr Birbiglia bought for £42 in May 2016.
The other was an orange scarab-beetle gem he bought for £170
The museum believes Dr Higgs, who has been dismissed, had sold items to at least 45 buyers on eBay.
Danish antiquities dealer Dr Ittai Gradel, who first alerted the museum to thefts, has tracked down artefacts that were sold to buyers in several cities - including Hamburg, Cologne, Paris and Hong Kong.
Some of the gems which he bought himself in good faith, and then sold on to another private collector, ended up on display in the Deutsches Edelsteinmuseum in Idar-Oberstein, Germany
One gem is thought to be a rare, 2nd Century head of the Greek hero Hercules made from obsidian, a type of volcanic glass.
Estimated to be worth thousands of pounds, the gem was one of the prize exhibits and appeared as a full-page photograph at the beginning of the exhibition’s catalogue.
In 2017, Dr Ittai Gradel paid £300 for the item, which was offered in a private deal under the alias Dr Higgs used - Paul Higgins.
Alternatively: "absent," "misplaced," "stolen," "gone," "removed," or "missing."