What is the difference between a primary product and a secondary product?

A primary product is the thing your company was formed to sell. If you're a Harley dealer, your primary product is motorcycles. You sell new Harleys, used Harleys you buy from estate sales and banks, and other-brand motorcycles that were traded in on Harleys. You also sell Harley parts, Harley accessories and maintain a Harley repair shop.

Your secondary products are the things you sell but you didn't necessarily form the company to sell. Harley dealers all sell t-shirts with the dealership name on them--these are very popular among Harley riders. They sell coffee mugs, piggy banks shaped like gas tanks, magazines, just all sorts of stuff like that. No Harley dealer goes into business to sell coffee mugs--if all you wanted to do was sell shirts and coffee mugs you wouldn't keep the inventory of $20,000 motorcycles!--but they've all got them. Those are secondary products.