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@juboo87 wrote:

can I not list best offer for specific amount of days.? 

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Non-auction single Buy It Now listings are "Good 'Til Cancelled"; i.e. the listing stays live until it is purchased or it is cancelled by the seller. The listing will simply renew every month until one of those things has happened; this includes a fixed-price BIN listing with Best Offer enabled.

You can create an auction with a 7 or 10 day duration and add a "Buy It Now" price; is that what you were trying to do?