My Dad now in his 90's has had a little kick of removing all the screws, bolts, and nuts from appliances and other stuff we throw away. We now have several jars of small nut and bolt mixes that we dig into quite often.
But for bicycle fenders I would stick to getting the exact right fit and length in Stainless with Lock Nuts. You don't want some odd ball fastener made of Chinesium that will have unpredictable time of failure stats...
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No matter where you're at... There you are... Δf:=f(1/2)-f(-1/2)