When did the memo that says we should be KEEPING cookies go out, and why didn't I get one?
I still delete my cookies every time I login, and never had a problem. 
If eBay does like to see their cookies, wouldn't they also like to see the same IP associated with said cookies? If his IP is changing every time he logs in that's just as suspicious as them not recognizing his PC.
OP, are you using Firefox? Since no one asked.
Try setting up an account at a local library and see what happens. If you have no problems it has to be something with your IP.
The bottom line is, I could sign up using info that I know for a fact is suspended with clean cookies/IP and last longer than you did, so there has to be one huge red flag that's giving you away. My bet is your IP.
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