Use entity relationship to exclude records on view I have a content type, lets say PRODUCT
I have another type, PURCHASE, which points to PRODUCT with an entity reference

It is simple using views relationships to get PRODUCTS that have PURCHASE relationships, get totals, all kind of groovy things...

BUT

How do I get all PRODUCTS without any PURCHASEs? So create a view with a relationship "Entity Reference: Referencing entity" but exclude all those that are referenced by a PURCHASE?

Contextual filters have that nice negate checkbox, but that does not help me under relationships.
You should be able to add a contextual filter, negated, that uses the relationship that you want.