Because the pewter will be of various consistencies (each company has their own recipe, and that will vary over time as well) there is no way of knowing what metals, and what proportions of them, your pile of scrap is made up of. It will be tin, maybe lead, zinc, allsorts.
That makes it hard for a manufacturer to buy as they won't be able to trust it and it may damage their moulds if they get it to the wrong temperature, or it may result in miscastings because it doesn't flow in the way they would want it to, etc., etc.
Your best bet is to use it for
raw material for drop casting your own metal models. Given you own a large pile of
raw material, the cost of buying a crucible and some moulding latex is possibly a worthwhile investment. The kinds of variability in metal quality I mentioned above are not important for drop casting, whereas in the kind of centrifugal moulds larger manufacturers use they are more important.
A good place to start investigating casting for yourself is
http://www.princeaugust.ie/