It would appear you bought on ebay using the guest option,so the item won't show on your registered account.
You say you want to return the item for a change of mind,and if bought from a private seller there not obliged to accept returns,where as a business seller must,but you will have to pay any return costs.
Look for the email from ebay you will have received when you paid for the item as that will be what you need to use to proceed with a return
Also see this link,Get help if you bought as a guest | eBay
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