How do you "buy" HN? HN is a community-driven site and long-running experiment. I suppose YC could sell the domain name, but the community would just evaporate and move elsewhere over night.
And in the meantime, reddit's community was completely unaffected by the changing of hands.
Last I heard, CN still owns reddit even though the site now operates independently, so CN still has the chance to recoup their investment, but this will take a more profitable business model without sacrificing user trust. Valuations on sheer page hits or user counts are faulty with a capital F U if you can't figure out how to make money from them.