I work in the IT industry for many years. Believe me, there is no way they can track you other than by your IP address and cookies. The user-agent wouldn't be a reliable method. Too many people use the same browser these days.
If they will make you verify your phone line every time you clear your cookies, that will:
1. Cost eBay millions in phone fees. Not a good business idea.
2. Cause hundreds of thousands of sellers, who clear their cookies every time they close their browser or restart their computer, to have to verify their phone line.
In short, HUGE problems for eBay. It's not going to happen, forget it.
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