When a business registers to do any business with the Canada Revenue agency (CRA), such a registering for the GST/HST, the business is assigned a Business Number. This 9 digit number is required by eBay for Business Registration. This number will be displayed on seller to buyer invoices so that the buyer can be assured you are collecting GST/HST taxes for legitimate business purposes.
For more information about Business Numbers, please go to the Canada Revenue Agency's Business Registration Online Web site.
If you don't have a Business Number currently, you can register for a regular individual eBay account and upgrade to a Business account in My eBay when you are assigned a Business Number.