Hi Patrick,
To identify Core products, it will be best to do it at Source Data that you sync to Phocas (can be excel) and then add it as a subdimension of your Product. It will then gives you the flexibliy of maintaining the Core products list.
Then you can focus on Core to get product list and continue to use standard Phocas measures.
For the measure, you can then use the Case When (similar to If logic in Excel) in the following post :
Eg: If sales >0, then ! else - to give you an indicator of those customer that have or have not purchase the core products.