I don't quite know exactly what you mean.
TO BUY SOMETHING
- You can create an account at the top. The signup button is right above the search bar.
TO SELL SOMETHING
- You can sell something by clicking sell your stuff at the bottom of the home page, under the section titled "Make Money with Us". That will give you a step by step instruction guide to posting things on Amazon. The Amazon Corporation will charge you to sell on their website. I believe it is currently $39.95/mo. but you can check that under that section.
-Hope this Helps-