It's a retail term, not specific to hockey equipment. Keystoning is when the markup from wholesale is 100% or doubling so if a product wholesales to the retailer for $100 then the retailer would charge $200. If that same item is keystoned by the manufacturer then their production costs would be $50 for said item.
https://www.thebalancesmb.com/keystone-pricing-in-retail-2890192