Dunno what a "pure" buyer is.

But a "churn" rate when talking ecommerce is something to with regular buyers leaving a seller as I understand it

How this applies to what this is about I'll never know.

Maybe ebay likes to talk dirty.

To my way of thinking -  a ' Pure buyer' would be someone who buys only and not sells as well 

Churn is a word that was used by internet providers . People who ' churn' were people with one service provider and went to another.

So to me that could be 1 of 2 things..... Someone who is pure buyer and starts selling or someone that has started selling on ebay from another auction site.

Could be miles off but that my take.