Did you make that purchase using the same account you are currently using with the app?
If so, you should be able to see that order in your Purchases, unless eBay has removed the item.
If you purchased with a different account, sign out and sign back in to the other account to see your purchases.
If you purchased as a guest (when not signed in to an eBay account), see here:
https://www.ebay.com/help/buying/returns-refunds/returning-item/returning-items-bought-guest?id=4065
