If someone wants a fleece to be washed before they buy (usually for export) I wash it in cold rainwater. This removes any dirt but leaves in the lanolin, which is consistant with what Customs wants. Otherwise as Jaykay says, just skirt the fleece thoroughly by pulling off any dags or contaminated bits around the edge, and any large amounts of VM. Don't go crazy about it though as it will take up a lot of your time which you won't recoup from the selling price. Just do enough to be able to present an attractive fleece for the buyer, who can then do most of the prep him or her self.
Mostly I sell my fleeces skirted but not washed, but if I do have to wash them then I charge extra.
To wash in rainwater, I use an old dairy trough which has a drain hole in the bottom. I place the fleece onto clematis mesh (but I love your idea of the plastic washing basket Jaykay
), dunk it gently into the rainwater, leave to soak for half an hour, remove and change the water, repeat til vaguely clean. I sometimes put a drop or two of Ecover washing up liquid into the first wash, to help with the dirt, rather than the lanolin. I then spread out the mesh and carefully open the fleece to its approximate shape, so as not to mess it up too much, and leave it to dry, which can take a couple of days. We don't have a bath
so using the dairy trough is useful for fleece.