I inspect my all my lenses with a small LED torch because every lens look perfect otherwise and i wouldn't dare advertise any lens as mint, even if I think it was. It might be an overparanoid buyer, one with high expectations, or one who backed out. I'm not a frequent seller but I have sold a few lenses and this is the first return so I must be doing most things right. I'll just re-inspect the lens and forget about this incident...

thanks