Forum Moderators: DixonJones
I'm sure there are others...these are just a few.
Look at incoming links. If it already has some good authority links that's less work for you. If all the incoming links are from the sellers "network of sites" then that's not so good, especially if he decides to remove them after you buy the site.
Look up SERP results for keyword phrases the site appears to be targeting. If a product page is titled, "Blue Widgets in Anytown, Anystate ¦ Acme Corp" then do a google/yahoo/msn search for "blue widgets in Anytown" or "blue widgets in anystate" or even "blue widgets" if you think they may rank for that. Use the google frequency filter and change the date range for results to see if there may be a lot of upcoming competition hammering out pages that may soon overtake an existing ranking page. If the recent results show some strong looking pages you could be in for a lot of work trying to stay on top.
Check Google trends to make sure it's not a fad or slowly declining term. I have a degree in COBOL programming and that 2 word phrase doesn't even make the trends list....and the one word phrase COBOL is on a 3 year decline - not to mention USA doesn't even make the country list for that term.
I guess there's a bunch more that could be done but my lunch break is over now so I must go :)
Be especially careful of community sites because administrators/moderators may leave if management changes. Also, they may already have made arrangements with the seller to move to a new project of the seller, leaving you with an unmaintainable site once you have bought the site.