I've written in the past about the idea of a 3D printer as a money making asset, and I recently got a request to revisit this idea. So I am.
First of all, if you are buying a 3D printer with the intent that it is going to make you money, but you have no other idea in your head other than "3D printer will make money" then stop. You are going to make a significant investment, you are going to have a new hobby, and you are going to see, most likely, minimal returns. It's true that for me, between Etsy, MakeXYZ and 3DHubs every roll of filament, every upgrade and every repair, even some of the more expensive ones, have been paid for by my 3D printer. But I have never felt that I recouped the theoretically initial cost of my 3D printer yet (remember, I won mine), and I definitely haven't managed to save enough in the bank that I'd feel comfortable expanding with a second 3D printer.
With MakeXYZ and 3DHubs the business comes to you. Mostly. So you're at the mercy of other people's whims. If it's a slow week, it's nothing you've done. And there's little you can do to change it. With Etsy it can be a different story because you can control what you're offering. If you've captured a niche there's a chance you can make a lot of money, but again, take advantage of the fact that you own the manufacturing machine and keep your stock low. If you make a listing no one wants your store will flounder. I've had good luck and bad luck. When I first opened my store I had a TARDIS ring that a lot of people wanted, but over time demand has dropped off. Fortunately I found another niche in custom soap stamps, but that well seems to be running a bit dry lately. But through it all there are listings that just have not moved at all. It's a lot of hit-and-miss to run a successful Etsy store.
Now I'm expanding and building my business towards commission 3D modeling. That's not something you can do just by having a 3D printer. However having a 3D printer is a value add when I can offer a free 3D print of what I modeled.
So I guess my position hasn't changed much. Owning a 3D printer won't automatically make you money, which will probably become even more true if 3D printers become more reliable and cheaper in the future. You're probably better off learning to model and hiring someone on MakeXYZ or 3DHubs who's already taken the financial hit to print it for you until it becomes obvious that you need one. Then, when you're sure that you're at a disadvantage not having a 3D printer, shop around and dive in.