How do I know if seller is selling genuine cables?

No guarantee. Buy from reputable sellers with great feedback. Pay with Paypal and try them for three weeks, if they sound good and meet the described criteria as presented in the add, be happy and enjoy the music.

Plan B: Only buy from authorized dealers at new or demo pricing.

A fake does not mean that it will sound bad. The two do not go hand in hand. Think about it!

Fake boobs look great and many times perform better than the originals, and they cost more! ;~)