With Pay and Ship, your item will automatically be marked as sold 24 hours after the item has been confirmed delivered to the buyer.
If you sold your item outside of the Pay and Ship service you can mark it as 'Sold' manually.
Here's how you do it, Go to ‘My Stuff’...
For iOs, you'll see the 'Mark Sold' button on the right of the item. You can un-mark it the same way.
For Android, tap on the item you want to delete, then tap the 'More'' button at the top right hand corner and you'll find the 'Mark as Sold' button there. You can un-mark it the same way.