When you buy something, the person selling it to you needs to make sure it's ready for you to take. This means they have to pay for anything that needs to be done to get the item ready. For example, if you're buying a bike, the seller might need to put it together or make sure it's working properly. They have to pay for this, not you.

However, you and the seller can agree to do things differently if you want. If you both decide on something else, then that's okay too.