Its really on a case by case basis, why are you buying it ? Is it because the name is good, or it has a good backlink profile / page rank ?
If its Page Rank remeber page rank belongs to page, not the whole site so likely your homepage will keep the same pr, but unless if you keep the same url's for the rest of the site you wont get any benefit from their existing page rank.
If its for its link profile i would be careful here, its not as simple as it used to be what looks like a good link profile could contain lots of spamy links (again this is on a case by case basis.) I would do some due diligence on the site using http://www.opensiteexplorer.org/ , http://ahrefs.com/ and using googles link search feature (ie. link"http://www.domain.com"). None of the above will give you the full link profile, for that you'd need to ask the seller to see the Google webmaster account for the site, but the above will give you a good starting point.
If its just because the name is catchy or has keywords in the url you may be able to pick up something similar with a .co (or similar) domain.