I have a few questions:
- How did they get your credit card details if they used their eBay account?
- Is it a family member?
- Have you ever had control of that other account?
- Have you ever given your card details to someone to make an eBay purchase?
- Did you sell a computer/laptop/mobile without wiping the memory?
If it is unauthorised use of your card then go to your bank and do a chargeback.
Also, I'd be asking the bank to cancel that card that was used and get a new one.
