It depends on the current traffic, if it's making money, if it's banned from Google, if it has potential to grow income...

Of course - assume its making money but revenue took a dive since Penguin, though the asking price is appropriately discounted.

Potential and any lurking bans/blacklists are the things then.

I can assess potential - any way to confirm a ban or de-indexing or something?

To see if it still indexed in google type in site:www.site.com (that should be in the search engine), if its ranking number one for yahoo or bing and is still in google index but no where to be found its been hit, but with some hard work you can change that, just start building links use more natural anchor words i.e website, visit, web page, blog, blogs, home, site, online, also a good one that I have found is the url and branded anchor words. Do not use spamming sites try and use 2.0 sites or pr1 or higher.

Google is the Biggest Search engine but not the ONLY Search engine there is .What you should concern about is whether or not it is making money.

You could build a list and send traffic to it in other ways, but make sure you have alternative traffic sources.