Go on eBay, search for the Nvidia Paragon voucher code or whatever it's called, and in the results select the sold listings filter. You'll see what others have sold their Paragon code for and you should then try to sell yours there at a similar price or start an auction at a bit lower. That's how I used to sell my unwanted used games and new digital codes. You also have a much better chance of selling it there than here.