I mean, I do the same thing with chainmaille. Any money I make on a necklace goes into buying more silver for more necklaces, but that's still a net gain. I took 40 bucks, bought silver, made a necklace, sold it for 200. Now that 200 is 200 worth of silver rings, but that's still a profit of a pound of silver. Whether or not it stays in the same form of currency doesn't have an effect on it being net gain.
Your selling cables has allowed you to buy more cable materials, so your net gain is cable materials.
And no, I don't think it does matter. The line between hobbyist and vendor is a blurry one, but that line isn't "profit" so it's a moot point. I think the line comes down to output. If you're making a couple keycap pullers and cables a week, there's no way you're a vendor. You're doing it as a hobby, and making a couple bucks.
If you're putting out 20 cables a week, well, we might have to define our terms a bit more.