I, obviously , wasn't the "seller" to cindilou, but I do sell alot of birds, and I think a more reasonable train of thought should be taken here. First, the moment you noticed the lice, you should have contacted the seller. At least then, the seller would have been given the opportunity to tell you how to treat the birds, or could have offered you a refund; also, the seller would now also be aware, and could have treated their own flock. Lice are a common and recurring problem in poultry. I would rather assume the seller was unaware that the birds they sold you had lice. Lice multiply rapidly... yes, a true "infestation" should have been noticed by the seller.
Though all of my birds are on a monthly parasite prevention plan, I HAVE found lice on them before, and of course, treated it immediately.
Just my .02