It means "let the buyer beware", which isn't the same as the buyer be aware. It is a warning to the buyer to remain on the skeptical side and if it looks too good to be true, chances are it is.
Greed and evil are not synonymous, neither is an admirable trait but evil is different. Evil is it's own intent and often can be the only gain of it.
I do think you'd get a lot of disagreement on that taking advantage of people is in itself evil, especially if it isn't illegal. That's like discussing the concept of ethics with a lawyer. Ethical and legal have fuzzy lines with some of them, particularly the corporate lawyers I know.
The thread is about how people who scam people, which I consider illegal just by the use of that word, can live with themselves. And I don't understand it either because I have that gene that tells me that's wrong but as we know some don't have that gene in any form. How can some become serial killers?
This thread speaks to a much deeper discussion about the existence of a conscience in some people. Wanna get deeper, the existence of the most mysterious part of human beings, the soul. Does every human being have one? Are there evil souls? How do we explain the heinous acts carried out throughout history against our fellow humans? Hell of a lot more than scamming them.