Remove sold items

Unique items sold via PayPal can be automatically marked as sold.  This is a convenient way to prevent a unique item from being sold twice.
 

Check the item as unique


When the For sale and Price fields are switched on, there should be an Is unique checkbox next to each image. Check this.


Edit on January 21, 2020
It is no longer required to set-up PayPal Instant Notification within your PayPal account when selling items directly via PayPal.  Sold items checked as unique will automatically be marked as sold.
 

Items sold via Mals-e checkout


If you use Mals-e.com (for adding shipping and tax) for site-wide checkout (instead of only PayPal) the PayPal Instant Payment Notification option will need to be enabled within your PayPal account like so:
https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNSetup/
When setting this up, you will be asked for an IPN URL.  It is:

https://yoursite.com/payment-notification

Replace yoursite.com with your actual domain.