Gazza, perhaps you could put a note above the feedback box describing what sort of thing to put, so you could mention in there about how long the goods took to arrive, were they well packed, were you happy with them, was communication good from the seller/buyer, etc.etc.
I can see where you're coming from, but I think if people have too many options it might put them off leaving feedback as they might think "I'll do it tomorrow" and then never get round to it.