A calculation like this must partly include some assumptions, which makes it important to be transparent. It is built on a best case scenario based on the question:
”How much pollution can potentially be saved annually through second-hand trade, if each used product replaces the production of a new one?”
This is why we speak of the ”potential effect”.
To be as clear as possible you can read all about how we did it, and about the method. There is also some clarification that will explain common questions.