Most of the parts are either supplied gratis or at a reduced cost for the advertising exposure. Most likely they use new hardware. A lot of hardware kits come prepackaged in small bags. Windshield trim might be one bag, bumper bolts in another. When I tear a car down to restore, I buy an extra box of baggies and use a marker to say what the parts are from. If I'm rebulding an engine for instance one bag will have the valve cover bolts and another will have water pump bolts. A sharpie marks the bags just fine. That way you don't have to hunt for one missing bolt that got mixed in with some others.