Well, half of the time when websites reckon Opera is incompatible, it's because they don't know what they're talking about. Go into the preferences and then network settings and tell Opera to identify itself as MSIE 6.0. I'll almost guarantee that it will work after that. I know that GMail won't display everything when my Opera (8 beta) doesn't identify itself as IE, but when it does, it suddenly works properly, just like it would have anyway. :lol:
EDIT - Just saw Effy's post. Well, I guess Ebay has a more indepth check than GMail. :-? I guess you'll either have to use IE itself, or upload somewhere else.