Let's say you're a car dealer, selling cars.
When a customer buys a new car, he often trades in his existing car.
What data model makes more sense?
A) create a sales order record for the sale, and a purchase order record for the trade-in, and link them together (related_order_id)?
B) create a sales order record for the sale, and a sales order adjustment for the trade-in?
C) something else?