Best ferret parent rule: DON'T use any Marshall products for your ferret at all. They all tend to be full of either carcinogenic ingredients (leads to cancer over time), Essential Oils (toxic), contain too many indigestibles (leads to insulinoma), are full of sugars (leads to insulinoma), etc.
If I recall correctly, the Marshall Ear Cleaner contains Eucalyptol, which is/is comprised of Essential Oils. So no, don't use it.
I'll let others chime in on what to try and do at home. When my ferret had an ear infection we took him to the vet to get checked out and get something for it.
If he had an ear infection he needs vet treatment asap for antibiotics. Left untreated it can travel to the brain.
Ferrets: Contessa Kitties: Watson, Oskar DIP Sinnead, Vincent, Boris, Zeus and Athena, Willow, Mr. Frodo, Indie, Lucrezia, Judge, Odin, Miss Emily, Suki, Cody, Aristotle, Butterscotch, Frankenfurter. RIP Herne, Ligeia, and Mr. Stubbs