Steve Gray of VS Tools has a nice query for GP to find all of your customers who haven’t bought a certain product.
Mark sure you check out Steve’s site.
Query for Customers Who HAVEN’T Bought a Particular Item
Payables & ReceivablesSQL & DataSales, Purchasing & Inventory
Archived post from 2008. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.