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Why doesn't the sales history propagate between old and new items?
What happens to the history of an item when relisting
Once an item is relisted, it becomes a new item and does not contain the history of the old ItemID. You can still look up the "old" data by using the old ItemID and make a GetItem call to look up previous history. If you needed that data for some reason for the new listing, you would have to post that history somewhere within the item (description, application data, etc) depending on what you were looking to do. The old link to the item on eBay.com will also work for an unspecified amount of time. The GetItem call will also tell you the RelistedItemID.
Answer Title: Best practices for relisting items
Answer Link: https://developer.ebay.com/support/kb-article?KBid=1542