The blogs selling those links likely sell too many links to stay off the bad neighborhood radar for very long, link buyers tend to want PR to flow to their less than stellar sites.
Links purchased on pages long after they are published likely don't carry the same weight as links that existed when the page was first published (discovered by search).
With those two things in mind, and since they are blogs, you may be better off sticking with a comment or two on pages related to yours that seem to be well indexed. Don't worry about nofollow, I suspect search engines ignore the nofollow tag in time as they figure out a site is worthy anyway. (don't flame me if you disagree)