Why can net working capital be negative?

Some businesses will take an initial deposit from the customer and deliver the goods or services at a later date or over a period of time. Instead of having the customer pay on credit, which would go to accounts receivable, you receive the deposit and end up with deferred / unearned revenue since you still have to deliver the good or service.

One example of this is the mattress industry. When you go to a mattress store, you are essentially looking at their showroom. The mattresses on display are not the ones you actually buy, they are just there for customers to look at and try out. When you buy a mattress, you are paying upfront for a mattress to be delivered at a later date. The money that the company receives is deferred / unearned revenue, since they have not delivered the mattress yet.

The company will then go to their supplier and order a mattress, which will ultimately be delivered to the buyer. However, the company usually buys the mattress on credit, so accounts payable goes up. In this way, the company has negative net working capital since it has accounts payable and unearned revenue, while having a much smaller accounts receivable balance since most customers pay upfront.