One area that 3D printing will change the game is with products that are not manufactured on a mass scale. Let's say for instance you are in the business of repairing household appliances like refrigerators or dishwashers. There are hundreds of models out there to stock parts for - or you pay an exorbitant price sourcing it on eBay or some large supplier.
But what if you could manufacture the part you need on demand? Simply print out the gear or pulley you need, or a spring mechanism or a belt. It's as easy as sending the file of the part you need to the 3D printer. In my company, we have a large number of field technicians across the country working on many different models of printing and scanning devices. We envision the day soon where the technicians will carry a portable 3D printer with them and will be able to produce the replacement parts they need onsite so they can complete the job in one trip. Or they can have it printed out on the client's 3D printer. No more waiting for costly overnight delivery of parts. No more maintaining costly inventories of parts that eventually need to get written off.
You are absolutely right. I keep thinking of all of the knobs and such in my older cars that need to be replaced. Soon I will go to a “print shop” and ask for a new door handle for my 2000 Avalon and I will have it within minutes.