If what it says on the blog is correct then he is using affiliate links for amazon for similar products to what you search. At it's core it is the most ethical

If what it says on the blog is correct then he is using affiliate links for amazon for similar products to what you search. At it's core it is the most ethical a free service can make money (aside from donations). He isn't selling the data or showing personally targeted ads. (Of course it could be using some amazon plugin that does it anyway for convenience or from ignorance, but he can do it without it through amazon apis)