Usually, Opera will go to the same site, but not always. The solution is to add "ebay" - or any other word you like - as a keyword. Bookmark the site first, then in the properties, add the word you want in the Nickname field. Then you can type just that in to go straight to the site.
You might even just use "e", or "eb"!
It's a superb idea, and is supported by Firefox too. I visit many regular sites this way.
Alas the two browsers don't always agree on which site to go to. I am guessing it depends on a central database that has been agreed by each company involved. If a company haven't paid Opera anything, they won't get the link. After all, it's a great way to control where the user goes to.
Think of a simple word like "instrument". Why should that go to a specific site about machinery and not a musical shop? Because someone must have paid for it to happen. (Correct me if I'm wrong.)
So unless browser makers can agree on using the same database, they will always differ. That's why for certain sites it's best to set them up as bookmarks yourself and choose the words that work.
Besides, it's much easier to invent a shorter word like "wbm" instead of having to type in full the word "waybackmachine". Plus it'll be the same in Opera and Firefox, and any other browser that allows nicknames/keywords.