Say you are selling posters.

The first idea is to make them slightly unique by limiting every image to X instances. This is not unusual in printed art.

The next idea is to let the buyer, at a higher price, limit the instance to 1. Then the customer will have a truly unique poster.

Now lots of posters are pebbles or leaves. The third idea is to have lots of such images but only present a fix number, say 10, to choose from. Every customer can choose between a numbered or a unique image. If a unique is chosen, remove it from the store and present a new one for next customers.