So I've tried this a while ago. Here is my short experience (only did it for like 2 hours).
AC Unity + Far Cry 4 had just released (around that time).
Looked up forums and gaming websites with open comment section. Posted a link where one could "buy it cheap". It looked like total spam to be honest, was surprised people clicked on it (was more of an experiment for me anyway).
Made €15 to date (once a person joins, you keep earning from his purchases), not bad for 2 hours of work of which
at least 1 hour was researching sites and looking at how to approach this. You could do it a lot faster once you get the hang of it. In total 384 people clicked my link, 7 joined using my spammy link.
Pretty sure a lot more people are working with G2A nowadays, but there are still enough methods to monetize your affiliate link and earn a decent buck

.