I am not familiar with amazon API's. I am building web app and I want to get user Seller ID, Marketplace ID and MWS token when he/she register on my web app. On time of register he and she will redirect to this site where user can login and submit agreement and then redirect to my web site with information of Seller ID, Marketplace ID and MWS token.
The MWS FAQ describes both how to get your own IDs and tokens, as well as the process of adding a third-party web application to your Amazon Merchant account.