Under most affiliate marketing arrangements, advertisers only pay for converted leads. There is basically no way they can lose money or get a negative ROI with this marketing method. Each new sale generated may have a thin margin after the affiliate payment is made, but it’s possible to structure in such a way that eliminates the possibility of a loss.
Affiliate marketing is commonly confused with referral marketing, as both forms of marketing use third parties to drive sales to the retailer. The two forms of marketing are differentiated, however, in how they drive sales, where affiliate marketing relies purely on financial motivations, while referral marketing relies more on trust and personal relationships.