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:

Role	Methods available
Seller API Access	All
Seller API Product Access	GetProducts, ProductCreate, ProductUpdate, ProductRemove, Image, GetBrands, GetCategoryTree, GetCategoryAttributes, GetAttributes, FeedList, FeedOffsetList, FeedCount, FeedStatus, FeedCancel
Seller API Order Access	GetOrders, GetOrder, GetOrderItems, GetMultipleOrderItems, SetStatusToCanceled, SetStatusToReadyToShip, SetStatustoShipped, SetStatusToFailedDelivered, SetStatusToDelivered, GetFailureReasons, GetShipmentProviders