Based on the comments I'd suggest asking them to add you as a viewer of their google analytics stats so you can learn more about the search performance.
Log out of your google account, delete your cookies and then search google for keywords from their analytics to test their actual search performance.
Look at alexa.com rankings for the domain and check out other domain auction sites to see what similar domains cost.
For blacklists check http://www.mxtoolbox.com/blacklists.aspx, http://www.blacklistalert.org/ and http://domain-blacklist.e-dns.org/
You should also attempt to post links to twitter and facebook to ensure that they have not blocked the domain for spamming.