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How can I convert Store Items to Auctions items using the API like I can do using MyeBay?
Using the website, via MyeBay you can convert a Stores Inventory Format Item to an Auction by selecting the stores fixed price item and clicking the Send to Online Auction button. Alternatively, you can make it a Fixed Price Item by clicking on the Send to Fixed Price button.
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What actually happens is that the Stores item is ended and a new item is created.
To simulate this via the API, you need to make a call to EndItem and make a call to AddItem with the ListingType set appropriately. For auction items, set it to Chinese for single quantity items and Dutch for multi-quantity items. For Fixed Price Item, set the Listing Type to FixedPriceItem.
You can find information on the API calls on End Item and AddItem from the documentation