What is the difference between offerId and listingId?
An offer created using the Inventory API is identified by its offerId. When using getOffer, one of the fields returned in the response is the listingId, according to whose definition, "Listing ID is the unique identifier of the eBay listing that is associated with the published offer" What is the difference between the two? Also, is the listingId the same as the itemId present in the filter field of the searchCancellation call
https://developer.ebay.com/devzone/post-order/post-order_v2_cancellation_search__get.html#Request.transaction_id. If not, where does the itemId, come from?
The Inventory API is an abstraction layer over the legacy Trading API. When you manipulate an inventory resource, eBay's back-end converts these state-changes into trading API calls. For example, when you publish an offer, information from the inventory item and offer objects are combined as parameters for the AddFixedPriceItem call
https://developer.ebay.com/devzone/xml/docs/reference/ebay/AddFixedPriceItem.html The result of a successful AddFixedPriceItem call is an "Item" identified by an "ItemID". These are more commonly known as listings and listing IDs respectively. Listing IDs and Item IDs are exactly the same, they're just different names depending on the context. I recommend reading through the Trading API docs to learn what exactly is happening behind the scenes, even if you do not plan on using that API.