I've cleaned my greenhouse inside and out. I didn't fanny about with it and it was empty at the time. My suggestion is to arrange that the greenhouse is empty, that there's nothing too awkward with regards it's position (like it's not right up against a fence) and manage your customers expectations; to get it looking brand new you'd have to take it apart because of the overlapping glass panels, but it still can be cleaned to an okay standard.
Takes about 15 - 20 minutes for a standard 8x6 greenhouse wfp inside and out. It's no hardship really.