brinked - 10:50 pm on Mar 1, 2010 (gmt 0)
what you're describing sounds like a thin affiliate penalty and not a link selling penalty.
You are right in the sense that some thin affiliate websites seem to never get penalized, thats can be attributed to the fact that they are using methods to hide being a thin affiliate site from google, or maybe they were already penalized and have recovered from whatever penalty they were in.
I monitor several industries and there are 2 different thin affiliate sites that have managed to not get hit with penalties, they might get hit for a few weeks but then they always return.
Dont assume anything, just because you were selling links doesnt mean google penalized you because of this. Look at your site as if you were a regular visitor, do you see any potential problems that maybe google would have picked up on with a manual review?