> Amazon throws away the receipts (it'd be a nightmare to handle so they just... don't).

I'm not sure what you mean by this. Amazon does keep track on where a given unit came from so that it is traceable to the original source, and at least in Europe invoices are issued for the inventory supplies between the sellers (these are documented at https://sellercentral-europe.amazon.com/gp/help/help-page.ht... - may not be public).