Here is a list of links related to guest purchases:
http://ocsnext.ebay.ca/ocs/sr
Here is what I found for upgrading from guest to member:
http://ocsnext.ebay.ca/ocs/sr?&query=3254
If that doesn't help, you can call eBay toll-free by clicking on "Help & Contact", found at the top and bottom of every page. Choose Account>Register for a new account>Contact Us (at the bottom)>Call Us (also at bottom). You will receive a toll-free number and a one-time passcode. Make sure you have all the information in front of you, an empty bladder, and perhaps a beverage of your choice.
Calling during normal work hours in UTAH will increase your chances of reaching the US customer service centre, otherwise it is in the Philippines. You may find you are connected with a CS who doesn't seem to understand, or is unable to solve your problem. Be persistent but polite, and ask for a manager if you need to, or call back.
Good luck, and come back to these user discussion boards if you have any further questions. A lot of the people who answer questions here know more than the customer service reps.