If you need a piece of armor to slot in a shell, you can't get it from the market or make it for yourself (the schematic isn't available for that particular level) - You can only buy it from a vendor (and they don't tell you where that vendor is).
You made so many mistakes in 1 paragraph that I've got my work cut out for me but, okay here it goes:
1. You can get Armourings off the GTN
2. You can craft Armourings, Mods and Enhancements, and Crystals and Hilts and Barrels and Implants and Earpieces, and custom gear. This part really stands out since it makes me wonder if you've ever actually crafted in SWTOR
Heck, this is how I keep my alts geared as I level (Cybertech, Armormech, Biochem...) so... in this case you are really really wrong. I mean if being wrong was on a scale of 1-10, you'd be at 11. That's how wrong you are.
3. Finding the planetary vendor is easy, aside from Quesh ALL of them are on fleet. You ever go to fleet?
This nonsense goes on and on, until you reach lvl50, where, for some inexplicable cause, crafters are taken out of the picture entirely by the quest reward system. They don't even let you have lvl50 schematics for Armor, Mods or Enhancements. Somehow, there is gear that continues on from fifty and well beyond, but only if you participate in the Operations and Flashpoint content.
4. Crafters are not useless at 50, in fact pre-50 I always relied on my own crafts and never bought anything off the GTN, at level 50 I started buying crafted gear off the GTN (14 Augment Kits and Augments per endgame character). I also bought Prototype Ultimate Medpacs (better than wasting my exotech mats) and Exotech stims/adrenals whenever I ran out of mats. I sell +41 crit Magenta crystals on the GTN for 225k, about 1 a week (not much profit, but the market is fairly saturated as of late). Overall, I make about 800k/week on the GTN, and I'm one of the lazier crafters. If you're not making +500k/week then you aren't trying.
P.S: It is not my intention to insult you in any way shape or form, but we need to stop the spread of misinformation on forums, people are easily confused WITHOUT false info, adding false info to forums makes them really confused.