How do I know if seller is selling genuine cables?

Some cable manufactures (Cardas) offers a service to verify the authenticity of the cable. It will cost you more, but that is the price of peace of mind. Also, ask the seller if he has an original receipt. Look for good feedback and stay away from Ebay.

If they have serial number you could check with the manufacturer.

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.