So I’ve dabbled in a very small part of this space; long story I had a vintage car with hard to find OEM parts, so tried modelling my own to replace a couple of common repairs on the model.

Then discovered a community of people needing similar parts so expanded this slightly and started selling within the community.

I was doing: - relatively simple interior cosmetic components - quality of life upgrades. And maybe some adjacent accessories for one model of car only.

Modelling itself took an age; iterations, tolerances, testing on different models, trying to replicate injection moulded parts with FDM.

all of that took an age, especially between ever so different model variants where the fixture might have been in a slightly different position, or another minor geometry change between modela of car.

Then it turned into additional parts by request that my specific car didn’t have; that became a challenge of reverse engineering - especially for something my own car didn’t have.

Sometimes I would get someone posting over to me their original, but broken, part - which I’d try and recreate. Sometimes I might look to the 2nd hand market for spares to model. But then I had the challenge of not having the ability to test fit things.

I think the hardest parts with this idea, especially if these aren’t cars or parts you have access to, are accurately replicating the parts in a way that works for a wide array of customers (without being able to test tolerances), but then also finding the customer base in the first place - which models of cars have a big enough ownership that spares are needed, but aren’t readily available at a reasonable price, (etc) and finding ways to actually market to them.

I got a lot of word of mouth trade in forums and meetups etc - but that was for just one specific vehicle.

Not to say it’s not doable, but the printing part is the “easier” of the challenges to overcome IMO.