In a non-regulated industry (like securities or real estate for example) most of your legal issues will be around how the associated affiliate disclose their relationship to you.
In many jurisdictions -- including in the United States -- individuals providing testimonial support in exchange for compensation must disclose this relationship.
Disclosure will be your friend -- make sure that your affiliate properly present their relationship to you to the individual they "refer" to you -- and you will be in much better standing for anything which comes your way.
If you are in a regulated industry -- for example if your price-comparison website is comparing a product which the person that does the actually selling needs to be licensed -- then there are much stricter rules. Sometimes the word affiliate or referral will need to be avoided. Sometimes you can pay a marketing fee-- and other times you can not.