This isn't something limited to eBay. On some occasions when a page doesn't want to load, or loads improperly, the browser loads the page in it's most basic form: text. That's why you don't see pictures, logos, etc.
It could be a hiccup in your internet connection, something slow loading on eBay's end, or a variety of other reasons. Regardless, it should only be temporary, and often a refresh of the page is enough to set it right.
Edit: I just realised you said it happened on all of this seller's listings. Perhaps they have some specific "active content" (templates, pictures, videos, something along those lines) that is causing your browser to struggle to load the pages properly.