I've gotten conflicting advice on this. If an affiliate posts his affiliate link to my site is that counted as a valid quality back link ?
If it is a text link then it's seen as a link. If you use javascript then the search engines won't see it. Creating an affiliate program is a trick that many web sites use to get more links to them.
If it's a normal hyperlink that doesn't have the nofollow attribute, it's a backlink counted by Google. I'm not sure whether it'd affect a specific page, though.