Content is King. Not keywords. Keywords help promote a site with good, unique content.
If I were looking at a website to purchase, I’d look at the content provided and traffic trends. Alexa (although inaccurate) would be a place to start.
Now, if you already had a similar site and was looking to purchase to integrate/combine another similar site, that might be more appropriate for keywords/SEO research to help boost or optimize your own search rankings.