Your avatar fits perfectly to the state of ignorance you are in.
You have not bought credits.
Do a free credit offer.
Then you can sell on the public market.
Have you bought credits? You need to do so once before you're allowed to sell on the market. (Free surveys and such for credits also count as buying.)
This has been answered multiple times, wow, it even tells you you need to but creds to unlock the market, come on now.