I have tried to find the answer using Google, but I'm a bit confused as there are a number of eBay APIs.
GetOrders
command should be used?CompleteSale
command in the the Trading API the correct command to use?I'm planning to use this Rails GEM: https://github.com/ReverseRetail/ebay_client
1) Yes, use GetOrders to retrieve orders from eBay (Dont use getsellertransactions it is super buggy). Here is the doc for GetOrders Best Practices. You may also want to take a look at this article Order management using Trading API - GetOrders (I would set the set "Create / Mod TimeTo" time to 5 minutes instead of their recommended 2 minutes.)
2) Yes, you would use CompleteSale, you are going to need three bits of information the OrderID, ShipmentTrackingNumber, and ShippingCarrierUsed. You might also want to note that you cannot use the same tracking number for multiple packages, the API will error thinking you are trying to game the system.
3) If you want perform the trading api requests on a store that you do not have access to, you will need to allow clients to authenticate their store with your app. Documentation on that process is very detailed and can be viewed here: Getting Tokens