Sure, but if I post data to the blockchain (perhaps a URL to an XKCD comic) and someone else posts that exact same data another time, how would we even know eac

Sure, but if I post data to the blockchain (perhaps a URL to an XKCD comic) and someone else posts that exact same data another time, how would we even know each other posted it? At some point, it's practically impossible to search the entire block chain. Now suppose each of use are selling the 'first' post of that data. How does a buyer know that we're really the first? And so I'm saying that even claiming 'first' is practically impossible via blockchain. Even if someone knew about both of the instances above and could arbitrate between them, there might be a third, older instance yet.