I use a facial brush and warm water to clean eggs before incubating them. It is no more than a hen would do getting on the nest after bathing. You do not want to scrub into the natural bloom on the egg so stains are fine. Tne facial brush I use is very soft and I buy replacememts often since they are only a dollar at tne Dollar Tree.
I've purchased shipped eggs twice and the hatch rate was 1/12 and then 2/12. I later found out she washes her eggs. I know shipped rates are lower anyway but can't help wondering if the washing made it worse.