I have gotten headers off ebay, Theres things you wouldnt buy.. Valves, pistons , rods, wlabro pumps, turbo manifolds (cast iron is ok) there are just to many fakes on ebay but if its not super important (intakes, headers, exhaust (you can correct them) are all ok in my book) If its something important I go through the actuall dealer cause I have alot of connections and I know im getting the real deal!
I have never had any welds brake on my cars just some dont line up correctly nothing a welder and grinder cant fix tho