Actually, this probably isn't the best place to ask this sort of question if you're after a real answer, considering the general vibe for domainers around here.
Seriosuly though ,the reasons why you see sales like this is because they've see some guy on Oprah that made $30k on a single domain sale. He's then written an (e)book about how he did it and made it seem so easy, and then filled it with recommendations on how people can do the same.
He never mentions specifically why the sites & domains that sell are worth so much. It's usually either a premium domain, or a site that has ended up becoming quite valuable in terms of information or community.
What he DOES know is that other domains have sold for hundreds of thousands of dollars... so why not his? Surely a domain with THREE generic words in it is worth more than a domain with only ONE, right?
And that's how the DP end of the domainer market works.