An item is automatically marked as "sold" as soon as both parties accept and confirm an offer. Afterwards, it will also show the orange "Sold" label.
If you have sold it somewhere else:
Please note that items that were not sold via the Shpock offer function will not appear in your sales count on your profile. This includes items where you arranged a deal via the public questions & answers section. Furthermore, please be aware that once an item has been manually marked as "sold" it will be removed from your selling list and you will not be able to access it anymore.