All good advice so far, but I'll add that complete, honest descriptions of items and their condition are always appreciated, and help both you and the potential buyer, as do numerous photographs in helping to confirm condition (especially since one person's view of "mint" or "as new" is sometimes different than another's).
From my own point of view, I always shy away from equipment that is dirty or hasn't been well prepared for sale - unless, that is, it's very cheap. It takes a only a few minutes to wipe a camera or lens with a damp cloth and perhaps use a cotton bud or two to get rid of dirt and dead skin from cracks and crevices - but it makes such a big difference and gives the impression of well-looked-after gear... which, typically, fetches better prices