I think you are confusing different terms. The turnbuckle is the center part that you turn to adjust the rig. The threaded part that fits into the top of the turnbuckle is a swage fitting. You do not have the proper press to swage a new fitting. Nor should you use the hand crimps found at the local West Marine. This type of swage fitting actually squeezes the fitting around the wire at a very high pressure. So high that it fuses the wire to the fitting. No way to take apart and repair. So using the same wire would require you to cut the wire where it enters the fitting. If you have broken strands at one location, the rest is soon to also fail."
I used the in-store crimping tool at West Marine, and the crimps they sell, to swage a short cable on my boomvang.
Came unswaged, twice.
The first time it snapped I got to watch the entire boomvang assembly perform a graceful arc into the sky, back into the bay, and sink without trace.
When I went back they said "actually, our crimping tool is no good - you need to buy one of these tools to do it properly". If the tool is no good, shouldn't there be a sign on it saying so?
Did you mean not to use their tool, the crimps they sell, or both?
Clearly there is some expertise involved when doing standing rigging and while it should be possible to learn the pitfalls, this is something that has to be entirely right, the first time.