Do I need the Userid to make Trading API requests?
The Username property of the requester credentials is not needed for authentication purposes when a Request Token is used.
Some API calls refer to a particular user, such as GetUser, GetSellerEvents, GetSellerList, GetBidderList and GetFeedback. For these calls, a <UserID> property of the request object represents the user who's data is referred to in the call. If the <UserID> tag is omitted, eBay implicitly uses the user represented by the Request Token.
For example, the GetUser request below specifies a UserID property in the request object. Regardless of which user's token is used, this call will return user data for "tuser1". If "tuser1" was not specified, user data would be returned for the user represented by the eBayAuthToken property.