Change featured image

I currently have a custom post type setup I can have a featured image on the post. I can achieve this simply using the inbuilt featured image option on the post itself.

That perfectly working. However my question is I wish to have a button or a checkbox within the posts itself so when I checkmark it or click the button it changes the featured image into a pre-defined one that a default featured image.

An example is I have a post offering the sale of a toaster. The main featured image of the post is the toaster itself. Then someone buys the toaster and click my new button/check mark and it changes the featured image to a default pre-defined image that has a sold stamp watermarked on it.

I require this so I can still display the posts however show in the featured image that it has been sold.

I am not selling anything purely an example. Any help would be great.

Regards,

Gareth