Great question — answering as the UphillTech team:
I strongly disagree with Hamish. You do need to worry about this. If Google finds these links, yes, they will punish you if you don't use rel="nofollow". Placing paid links on your page (whether or not they are relevant) violates the Google webmaster guidelines. They are receiving a lot of criticism for a decline in their search quality so they are cracking down on anything that distorts the link graph (from their perspective). Those companies offering to pay you for the links will not be interested in links that are nofollowed because they have zero SEO value. Search for term "paid links" and you will find any number of articles on the topic. For example:...
Full answer by Greg: https://webmasters.stackexchange.com/a/9779 (CC BY-SA 4.0).
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