Only a calssified listing remains active for 28 days, Buy It Now and Auction listings only remain active for the time period you choose to set (from the set options available) or until a transaction is completed.
What you can do is list multiple amounts of a single item, so that will keep the listing live for the set run-time period at least. When the listing then expires, you simply go into your account and you will have the option to re-list once more. One click of the mouse and your listing will give you the option to edit if required, then if you are happy, just select the list button and the same listing will appear once more.
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