How do I know if seller is selling genuine cables?

Listen to 'em if they sound right LOL!

Fake cables might sound good too, that doesn't mean that you want them.
You could buy from me if I had anything to sell. I don't at the moment. That would be a guarantee. My cables are not fakes unless they are made so well that even the manufacturer missed it.

Only buy from persons with many killer feedback then pray.

"So I guess the answer is to buy from a reputable seller w the understanding that you have the right to return if the manufacturer says they are not genuine."

Wouldn't the obvious sub par sound tell you it's a fake?

Given how system=dependent cables, i am not sure that how they sound is a reliable indicator, unless you had another known genuine reference. YMMV.