What's good for one, is bad for another. Who cares what other people think?

I think, A bad guy meaning:only for your bad

There are such kinds of sellers, that's because there are such kinds of buyers. No market, no business deal.

You can answer for your motivations, the bad guy should answer for his own motivations. There are so many ways to be bad, if you really want an answer, you should pick one of those ways, and maybe something could be said about what drives to that choice. Anyway good and bad for human beings can be separated (most of the time) even on philosophical grounds, no need for religion here.

This depends on how one views being good, or being bad. If you don't rule/measure anything with an external standard, then we are all bound to have different and conflicting ideas.