You can read eBay's policy on stolen property here: https://www.ebay.co.uk/help/policies/prohibited-restricted-items/stolen-property-policy?id=4334&st=3...
You can try reporting it to eBay - the reporting procedure is being changed and I'm unclear how the new system will work.
However, for eBay to take any action a report must have been made to the police, who can contact eBay for assistance if required. They won't discuss reports of theft with users, for quite good reasons I think. Claims of ownership or theft can't always be taken at face value. These are issues for the police or lawyers to investigate, not eBay.
The difficulty of getting known stolen items removed has often led owners to bid on their own items, either to identify the culprit or simply to get their items back. I personally know of two cases where this was done.