Why can't I middle click on these links ?

I don't think so. This is just shitty website design on the part of the website developers. They aren't links, they are elements with click handlers on them, so the browser doesn't know what to open.

On some websites it works if you click one part of such an element, eg. the text.

Unrelated because it's a different problem, but if a website actually disables your right-click, try holding Shift while right-clicking.

Because they aren't hyperlinks to a website. When I click them, a popup window opens on the same page. I assume this is something done with JavaScript. But I have some addons and not sure if this is the intended behavior or not.

Can you do the text instead of the image?

If you use the "Inspect (Q)" option, you can see that it is not a hyperlink. JavaScript will interfere and act like you clicked on a link calling a specific JavaScript function.

You can't middle-click them because they aren't links. That is to say, they are not a elements but div elements with an added click event handler that navigates to another page. There's a case to be made for doing things like that on a website that's trying to behave like a native application, but Ebay fundamentally behaves like a website and building its navigation this way is bad design.

as a workaround, you could duplicate the tab a few times, and when you opened the item you wanted switch to the next one.

this only works as expected if the list will contain the same items in the same order every time the page is loaded.