So you went and bought a new printer, now what?

not sure I have made a profit but I am covering filament costs for sure.

I have my brain teaser puzzles I sell (both plain and customized), and I do custom work for a couple of clients - custom RC stuff - both car and plane.

What the niche is will vary depending on where you live.
Do you have an active cos-play community in your area? a lot of people do props and such for that.

The issue with those sites is most buyers are expecting injection mold quality at mass production prices. Many of them don't know what a 3D printed part will look like.

I have made maybe a couple hundred bucks over 2.5 years.  Compared to probably $1500+ spent on printers and parts in that time.  You will never make money printing trinkets for people on 3Dhubs, especially if you value your time and sanity.  If you are lucky, you will find someone with an ongoing project that has some prototyping needs and start an ongoing relationship with them.  I had one guy who was making a board game that brought me continuous business for 6+ months and bought several rolls of filament that he wanted to try out, and I kept 80% of those for my own use.  That is the only reason I am still on 3Dhubs, for the 1 in 100 chance that I run across an actual good customer who is willing to spend money and understands the limits of 3D printing.

Also sold a bunch of custom vases to this lady that was convinced there was a market for vases that could clip on the side of old people's walkers... in case they want to cart flowers around with them all day or something.

Sounds to me like there may be an opportunity for an independent operator to sell, not to the public but to design studios, light mfg, etc. Become a boutique not a hub. Develop ongoing relationships with people that may already us 3d services and understand what they are going to get. Sell yourself as a backup for their in house system for overflow or emergencies. Perhaps compete with their in house farm on timeliness and availability. Use a high quality and very reliable machine or farm of machines. Maybe eventually offer printing, scanning and laser cutting/engraving.

The catch 22 may be that you do indeed need a reliable, quality and presumably somewhat expensive machine to assuredly print good stuff every time, maybe have more than one machine if one printer goes on vacation for a bit. When someone comes to you for an emergency repair part or a prototype, they are usually going to be in a time crunch. Whether you can or cannot deliver the goods on schedule and under stress will determine whether you retain that customer or get new business.

But, it could be lucrative. those kinds of customers aren't typically your penny pinchers.

Umm...not necessarily "expensive". reliable, most defintiely... but both of my used SD4's are both reliable and put out quality prints day after day, hour after hour.. and neither one was "expensive" by any stretch of the imagination - even including the costs associated with the upgrades that have been done, they still each cost less than a lot of other so called "quality" printers out there.

#1 has been running strong with little more than basic maintenance for about a year - basic maintenance consists of cleaning & lubing the rods and adjusting belts. Once all the upgrades were complete, I have had to do very little with it other than that. And last summer it ran for 8-12 hours a day, at least 6 days a week for several months.

That said - if I were to include my labor in those costs, then yeah, there would be a big difference... but I am of the belief that any printer is going to need time spent in maintenance, repairs/upgrades, etc., eventually anyway, so kind of a moot point. I just happened to have done 98% of mine up front.

Printer cost (used SD4 in working condition) - $400
Rumba board & related (stepper drivers, USB cable, etc) $180
Better power supply $27
Bearings (12 LM8UU) & new smooth rods ~$60 (+/- a dollar)
E3D v6 hot end $80

Total spent $747

Plus the cost of filament to print the Lawsy Carriages & a few other items - still well under $800.