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 eBay.
It's a part of eBay that passes under the radar most of the time, but every now and then eBay members can find their IDs to be front-and-center in their eBay life, or the focus of various kinds of problems.
Here are some basic facts about eBay IDs that every eBay member ought to know and understand.
eBay IDs can't be email addresses, but some are. 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.
eBay IDs can't be a dot-com. 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 form 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.
eBay IDs can be associated with an About Me page. 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 their ID, meaning that they can change the name under which they're known to the rest of the eBay 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 and that this could serve as something of a red flag to other eBay members considering doing business with you.
eBay IDs can be searched for. 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, amongst other things.
eBay IDs can be bookmarked. When you encounter a member you'd like to remember (usually because you like what they sell and plan to buy from them in the future), you can save that ID to your favorite sellers list, bookmarking it for easy access in the future.
People may have multiple eBay IDs. There is no rule against any one person having multiple eBay IDs, or from their doing this without telling anyone else (including their buyers) about it. 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 IDs can be suspended. eBay members that have repeatedly or egregiously violated the length 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, there are processes in place behind the scenes by which it can be reinstated, usually involving some combination of time, documentation of identity, and the righting of wrongs.
eBay IDs can be cross-linked. 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).
Every eBay ID is associated with a 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 act on eBay as though you're being watched, by everyone.
Most of the time eBay members don't think much about their eBay IDs, but when circumstances dictate it, knowing the facts about eBay IDs can help members to make the most of their eBay ID or to avoid doing things that will result in suspensions or cross-suspensions.