Basically I manufacture whatever people tell me to manufacture.
Partial list of what I've done to date:
Cases for electronic devices
Replacement gear patterns
Packaging machine parts (I do a lot of these)
R/C car parts
Prototypes for inventors
Novelty beverage container holders (think cup-sleeves)
I can do anywhere from batch-size one to hundreds if that's what the customer wants. I can also make injection molds via 3D printing (this is a thing, and how I got started waaaay back) and make larger batches of things.
...right now certain video-game companies are looking to produce physical models of in-game characters/items/locations/ships/whatever. The big complaint with some of these online-world model games is your purchase in game has no counterpart in meat-space... you spent $5.99 on a grande-supremo-mucho-deluxe plasma-chainsaw for your character. It'd be nice to have a physical model of it for your desk. Or a physical model of your character for that matter.
The idea is to have a company produce these under secure conditions to prevent copyright abuse and distribute said gewgaws on demand. Say you want a physical model of Captain Wendy Rostok Prussa of the Starship Thunderstruck holding her +2 Cryogenic Flamethrower. Pay the game company... they send me the character data as an STL, I print it I ship it everyone walks away happy.
Got a long ways to go before that bears enough fruit to feed the family, but it's a start. I've laid the groundwork, maybe others will see it through. Point is I'm in my glory here doing what I love and it hasn't driven me bankrupt yet.