Your eBay ID is the nickname that you use to log into eBay, the one that other eBay members see, and that is associated with all that you do on the site. Here are some basic facts about eBay IDs that every eBay member ought to know and understand.
New eBay IDs Can’t Be Email Addresses
Way back in the eBay dark ages, members' eBay IDs were simply the email addresses that they supplied to eBay when they joined. So while new members can't opt to use an email address as an eBay ID today, there are still members floating around eBay with email addresses as their eBay IDs. There's no reason not to do business with them.
Web Addresses Can't Be Used as IDs
eBay has long maintained a policy that IDs aren't to be used for linking or explicit advertising, meaning that it's against eBay rules to have an ID that is a website address, phone number, or other forms of business advertising. If you see one of these, it could (though this isn't definitive) be a reason to look just a bit harder at a seller and their ethics.
However, eBay members can create an About Me page that tells other members something about themselves, their businesses, and their life on eBay. Whenever you see the word "me" in red and blue beside an eBay ID, you can click on the word to find out more about the person in question. You can also create a similar page for yourself; there is no cost for doing so.
eBay IDs Can Be Changed
eBay members can change the name under which they're known to the rest of the community. If you're unhappy with your ID, consider changing it, though you should be aware that eBay will mark you as having recently changed your ID. This could serve as something of a red flag to other eBay members considering doing business with you. Likewise, be wary of doing business with someone you don't know whose ID has been recently changed.
If you've become interested in a particular eBay member, whether as a buyer or a seller, eBay provides a member search tool amidst its advanced search options to enable you to find them again. By searching for an eBay member this way, you can contact them through eBay or find a listing of all of their current items for sale, among other things.
If you have a good experience with a seller, you can save that ID to your favorite seller's list, bookmarking it for easy access in the future.
People May Have Multiple IDs
There is no rule against any one person having multiple eBay IDs. This means that several eBay IDs that you regularly buy from could well be the same business or individual behind the scenes. It also means that you're free to use multiple eBay IDs as an organizing practice to help with your buying, selling, or accounts management.
eBay members that have repeatedly or egregiously violated the lengthy list of eBay rules may find that the eBay IDs in use when this activity occurred become suspended. After an eBay ID has been suspended, it can be reinstated if certain conditions are met, usually involving some combination of time, documentation of identity, and the righting of wrongs.
Cross-Linking eBay IDs
Since all of the different eBay IDs registered by a single person or business are in the same eBay database, it is easy for eBay to identify IDs that are "related" to one another. By matching mailing addresses, network addresses, phone numbers, financial accounts, or names and addresses used, eBay can even cross-link accounts at the organizational level (i.e. all IDs belonging to a given family or a given business).
In some cases, when one ID is suspended, eBay may suspend all of the IDs to which it can be cross-linked. Reinstatement is the way to go in this case; just creating another new eBay ID is unlikely to solve the problem (it will probably be found automatically and suspended relatively quickly as well).
Feedback Profile and Transaction History
Because eBay is essentially one giant database of buying and selling activity, and because of the way eBay's feedback system is designed, every eBay ID has a history, both of feedback about it from others and of things that eBay knows about it—average shipping times, purchase history, and so on. There is no anonymity involved in an eBay ID—so you should always behave on eBay as though you're being watched, by everyone.