In curl command you have to do like this,
curl -u your-app-id:your-app-token https://sandbox-api.flipkart.net/oauth-service/oauth/token\?grant_type\=client_credentials\&scope=Seller_Api
you will get the result like this on success,
{"access_token":"394b7d-418a-43f6-8fd5-e67aea2c4b","token_type":"bearer","expires_in":4657653,"scope":"Seller_Api"}
by using this token you can make further api call's like for example listing api,
curl -H "Authorization:Bearer your-access-token" -H "Content-Type: application/json" -d 'json-here-see-format-in-api-example' https://url-end-point-refer-api-docs
note: you have to create app id and secret in "https://sandbox-api.flipkart.net/oauth-register/login" for sandbox. Don't store the access token it will get expired in certain period of time.
link for api doc's - "https://seller.flipkart.com/api-docs/fmsapi_index.html"