Sebsandiego is one of many EBay sellers who do not operate as an EBay "registered business seller", but as a private individual, even though he/they sell really quite large numbers of items, and clearly make not insubstantial earnings that way.
Private sellers, particularly in EBay auctions, appear not to have any obligations to disclose contact information. However, under EBay rules, business sellers must display their full contact details and sign-up to a fair trading agreement - see the Ebay Know Your Rights Page if you haven't already found it.
I often wonder whether HM Customs & Revenue would be interested in such "private" EBayers, since in principle aren't they liable to pay Income Tax on their EBay earnings if they reach a certain level? ....... .T.