Print it when
- Nobody sells it — a discontinued or one-off replacement part.
- It must fit your exact dimensions or your specific boat, machine, or space.
- You need one to a few, fast, without minimum-order quantities.
- You're iterating a design and want to hold each version in your hand.
Buy it when
- A mass-produced version is genuinely cheaper and good enough.
- The part is safety-critical and needs certified, rated materials.
- You need thousands of identical units — that's injection moulding's job.
The grey zone
Plenty of parts sit in the middle: available to buy, but not in the size, colour, or material you want. That's where printing quietly wins — you get exactly the part, not the closest thing on the shelf.