Remove BUY button for sold items

Items checked as Limited quantity in the admin section will no longer display the BUY button when sold out.
 

Check the item as Limited quantity

When the For sale and Price fields are switched on, there should be a Limited quantity checkbox next to each image. Check this, and set the Number available.
When the available quantity of an item is purchased, the BUY button will no longer display.  If you are using mals-e.com for checkout, the additional steps below are required.
 

Items sold via Mals-e checkout

If you use Mals-e.com (for adding shipping and tax) for checkout the PayPal Instant Payment Notification option will need to be enabled within your PayPal account like so:
https://developer.paypal.com/docs/api-basics/notifications/ipn/IPNSetup/#setting-up-ipn-notifications-on-paypal
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.

NOTE: When using Mals-e.com, you must also use the "PayPal Commerce Platform" option within your mals-e.com account for the auto-removal to work.