When you post an item and it sells, just click the check mark to mark it sold.

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Once it is marked "Sold" it will add it to the topic title so other will know the item(s) have been sold.

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If your item for some reason is no longer sold or is for sale again, You can also go to your topic and click the check mark again to mark it as not sold.

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Hopefully this will help so other know when items are sold so they dont have to ask in the topic over and over as much.
This feature is only available in the classified section.
So please remember, if your item you have listed does sell, please mark it as 'Sold'.