What is a 'negotiable warehouse receipt?

A negotiable warehouse receipt is one that can be given, for value, to another.

You have ordered product from a manufacturer to be delivered to a warehouse destination. You receive a warehouse receipt saying the items are in the warehouse and you now own them. If you owe someone else money, you can give them the warehouse receipt and they will now own the goods, in payment, or partial payment, for what you owed them (the value).