Sometimes a person can have an item which is ageing, in a state of disrepair or simply wants to upgrade to another version. One can take advantage of tradeoffs whereby you can submit the old item to the seller in order to acquire a new one at a significantly reduced price after factoring the cost of of the old item.
I always trade in my old car battery when buying a new one. This saves me about 10%+ off the cost of a new battery. The trade off price is usually arbitrary and many sellers will try to profit from the trade in by giving a lower valuation for the used stuff. In most cases, you can get more out of your old stuff by selling them to direct users or other consumers.
