I have setup a landing page with a form. When people enter email, that email is assigned to a list called "non-member"
When the buy through the woocommerce membership plugin, they are assigned to another list called "member"
BUT since they are assigned via the landing page to the non-member list, they should be automatically removed from this list when purchasing.
There is no way actually to remove from a list when they buy, but it's a good idea, I'll submit it to the team. There is a similar implementation on "rules by category" panel, we could extend the concept to the generic "buy" action.