Its not rocket science dave...
1. Click this:
http://pages.ebay.co.uk/help/contact_us/_base/index.html
2. Selecting the most appropriate topic
(in this case
Account Security Report fake eBay emails, unauthorized account activity, or other safety concerns)
3. Click Continue
4.
Step 1: Select a sub-topic... hmm let me think.. Unauthorised account activity I think.. (select it)
5.
Step 2: Select a sub-topic.... how about.. Report another members' account stolen
6. Click continue
7. Click
Email us
8. Login to your ebay account
9. Enter the userid of the persons account which has been stolen and your comment about the car, how its a fraudulent listing, etc etc.
10. Click Send.
Not really that challenging. Its not a complaints system, its a security system. Its well designed and tbh has dealt with every eventuality when i've contacted them about items for sale on there.