eBay Inventory

The Inventory API is used to create and manage inventory item records, and then convert these inventory items into product offers on eBay marketplaces. The Inventory API has the following entities:

• Location: a seller must have at least one store or warehouse inventory location set up before that seller can start creating and publishing offers with the Inventory API. Every inventory location must also have a seller-defined merchant location key value.
• Inventory Item: before a product can be sold in an offer on an eBay marketplace, an inventory item record must exist for that product. An inventory item record contains such things as product details, item condition, quantity available. Every Inventory Item must also have a seller-defined SKU value, and this value must be unique across the seller's inventory.
• Offer: an offer is what becomes a live eBay listing. An offer is first created, and then it is published with the Inventory API. Each offer must be associated with an eBay marketplace, an inventory item, an inventory location, and a category ID. The offer will contain quantity available for the offer, the listing description, and an offer price. Every offer must also reference a payment, a fulfillment, and a return business policy, so the seller must be opted in, and have business policies available to apply against the offers.
• Inventory Item Group: an inventory item group is necessary if the seller would like to create and manage multiple-variation listings through the Inventory API. An inventory item group is a collection of similar inventory items that might vary on a couple of aspects like color and size.
• Compatible Products: the product compatibility calls allow the seller to create a compatible vehicle list, or vehicles that are compatible with a motor vehicle part or accessory.

Technical overview
The Inventory API is more-or-less a "standalone" REST-based API, but it does have the following relationships with other APIs:

• Trading API: sellers may use the Trading API to get some listing-related metadata, and may also want to use GetItem call to view the listing that they have created through the Offer calls.
• Account API: sellers can use the Account API to opt in to Business Policies, and to create and manage business policies (that are used/referenced with the createOffer call in the Inventory API). Inventory API users may also need to use the getRateTables call of the Account API to get the unique identifiers of shipping rate tables that they may want to apply to a fulfillment business policy
• Fulfillment API: sellers can use the Fulfillment API to get order details and manage shipment tracking.