3D Printing Practicalities ontact 
Learning Robotics
Protoyping
Repairs
Learning Computer Assisted Design
Niche Product Parts
Making parts for products.
Upgrade Parts
Reverse Engineering
Multi Material Models
Body Protection
Rapid Replacement Parts
Sell 3d Scans of Objects.
Can you make money?
3D printing scales but it takes quite a bit of room and you really need to have machines of known reliability like Prusa which cost more than your bargain basement entry level machines. Ideally they should be on a network and be running in a self contained mode so that one minimizes the chance of one point of failure bring down the whole system.
To make substantial income takes high value products such as items that are
no longer manufactured like replacement parts for vintage audio gear.
You can make fair money creating files to be printed on
3d printers providing you can find customers
that do not wish to invest in in-house talent.
The 3d printer is just a portion of the cost of being able to print items.
Software Acquisition
A Powerfull enough computer to make designing efficiently.
Scanning
Training on Cad Software
Time Spent Learning Plastics
Time
Patents:
If you have something that is patentable the reality is that your patent is worth nothing unless you can afford to defend it. Patents are routine not given out and patent attorneys are real money. Be it known that many countries are not in the least interested in protecting your intellectual property rights and those include the largest markets on the planet. One other thing to watch out for is that if your product could give an advisary of the USA an advantage your product can be banned from production or being patented.
Usea 3d print shop service.
Patents
Copyrights
UL Certification
Building Codes
Legal issues
Choking
Part Failure
Warranty
3D Printing Services
STL Sources
Cults ES
Printables
STL Finder
Yeggi
STL Base
Thingiverse
Thangs
Your Local Market
Be realistic. How many folks are actually creating or fixing things and just what are they fixing or making. Generally you need to be rubbing elbows with folks who are going to use your service.
Repairs
Turntables
Cameras
Music Accesories
Vintage Cars
Loudspeaker Parts
Prototypes
Prosthetics
Mailboxes
Etsy
Ebay
Files Sales
SomeExpertts
Thomas S
Chris's Basement
3D printing nerd.
Maker Muse