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If you are referring to your bumper you had listed for sale, I think you simply update the thread as Sold (you did) and contact the moderator so they can close the thread to help aware viewers the item has expired:
Not 100% sure of the procedures on this site, but I don't think the Administration allows, or will, delete threads in these cases. Having a copy, in case either party has a transaction issue (especially for long-term to record any seller issues), is probably one large reason why.