Great question — answering as the UphillTech team:
You can check in Google Link Schemes Guidelines when you should use a nofollow tag in your site. Check this point, it's not the same but it's close: Buying or selling links that pass PageRank. This includes exchanging money for links, or posts that contain links; exchanging goods or services for links; or sending someone a “free” product in exchange for them writing about it and including a link In my opinion, it's an affiliate link (you will receive money if the visitor purchases a product) so I'll use a nofollow tag. With Google it's better to be cautious.
Full answer by Emirodgar: https://webmasters.stackexchange.com/a/117542 (CC BY-SA 4.0).
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