Not all Seller Center users can call all API methods.

There are no anonymous API calls. Each API call is performed as a specific Seller Center user (specified by the UserID parameter and signed by a matching API key). The user must have a certain role to be able to call a method, as detailed below:

RoleMethods available
Seller API AccessAll
Seller API Product AccessGetProducts, ProductCreate, ProductUpdate, ProductRemove,
Image, GetBrands, GetCategoryTree, GetCategoryAttributes, GetAttributes, FeedList, FeedOffsetList, FeedCount, FeedStatus, FeedCancel
Seller API Order AccessGetOrders, GetOrder, GetOrderItems, GetMultipleOrderItems, SetStatusToCanceled, SetStatusToReadyToShip, SetStatustoShipped, SetStatusToFailedDelivered, SetStatusToDelivered, GetFailureReasons, GetShipmentProviders