Google Ranking Factor – Affiliate Links
Similar to the link placement in header, footer or any widgetized or template-based areas in a website, you should avoid gaining or using followed links from affiliated sites or businesses.
These are blatantly unnatural and should always carry the nofollow attribute.
A very small amount of use of followed affiliate links may be OK subject to them being a relatively small percentage of your link profile. If in doubt as to what that percentage might be, it’s best to nofollow the links.
You might wonder how Google can detect what is or isn’t an affiliate link – and well, this might be hard to really figure out for them, but if you know it is, no-follow it.