Please use the listing text and photos to assess a particular item that you are interested in rather than my categorization - which is of course a subjective assessment.
But here's how I would define these terms:
Excellent - An obviously used item that fully works and which looks close to new, particularly from the angles that are viewed in normal use (so it might for example have scratches on the underside). Any blemishes are small. Careful touch up of paintwork (usually in the case of black items) to restore full visual impact.
Very Good - Between Good and Excellent
Good - Fully working. Some signs of wear. Knobs/lamps/controls all present. Some blemishes but no major dings or scrapes. Fair - Substantial signs of use but fully working. Some dings and scrapes - see item images for details. May be missing some trim components, minor screws etc. Ugly - Usually fully or mostly working, but check the listing text for actual details. Cosmetically poor. Ideal as a donor unit for good parts to renovate another unit, perhaps that's in better cosmetic condition. For spares/repair - Not working (in full or part), or untetsted. Usually sold with no warranty and with no right of return for lack of functionality.