Aftermarket (merchandise)

Aftermarket merchandise is like when you buy a toy and then decide to add extra parts to make it even cooler. It means that someone other than the original maker of a product is selling things to add to it or change it. Like if you have a car, and you want to make it look different, you can buy special lights or wheels that weren't made by the car company. These things are called aftermarket merchandise. It's like adding toppings to your ice cream, it makes it even better!