Well, at least in the US...
Well, at least in the US (I saw a case on one of the numerous TV court shows about this kind of thing), you can sell a deed or title and any and all rights that deed or title gives you. Note "any and all rights" may mean no rights at all. If you claim the deed or title actually confers any rights and it doesn't you've commited fraud. If you're careful to not make any claims, you're merely a huge scumbag but technically not a fraudster. Or possibly misinformed or (probably in this case) crazy, if you honestly thought you had clear title to something but actually didn't. (In the court case, the person was somewhat greasy, the deed was some 1800's common-law "land in the middle of the desert" sort of deed he had ininherited, that may or may not have still conferred any rights whatsoever, and he traded it for a car knowing this. The judge made him undo the deal.)
In other words, EBay should definitely block any claims that someone is selling the sun since they are fraud, but she may be within her right to sell a deed to the sun as long as she doesn't claim it gives her clear title or anything... although hopefully nobody will be dumb enough to bid on it.
That said, I think this is in the same category as the people that were selling "exactly what you see in the photo", showed a photo of a Playstation or XBox box, and let people bid an empty cardboard box up to $1000 or whatever. Hopefully, the Spanish court will not clock EBay for blocking this, and hopefully will charge this crazy old bat EBay's legal fees. edit: Excuse me, I mean crazy allegedly old bat.