Not all gear are in equal demand, far from it. The demand for gear ranges from just 16 pieces (Nubian Design Tech Mk1 prototype) to more than 9000 (Carbanti Mk3 salvage).
Here's how much you need of all possible gear if you want to fully equip all 92 characters (current as of September). You might not reach there in this lifetime, but it gives you a rough idea of what gear is in high / low demand.
Then here's where to obtain the most demanded gear pieces, trying to optimize grinding for multiple hard-to-get gear at once.
For instance, if you need Sienar Holo Projectors Mk2, it's better to hit node Dark-Side 7A (at 10 energy) than 3H (at 6 energy), because you get a chance to get some other high-value gear there as well.
Note: always buy everything you see from the Shipments Store (at 800 - 2700 credits), because you'll need them at some point, and that's really cheap.
Note 2: This selection doesn't take character shards into account. But in the end, I found that grinding gear is way more tedious than grinding characters, even the ones that have only a couple of hard nodes.