1. You get your Legacy when you finish Chapter 1 of your story. Should be around level 35. After finishing Tatooine and Alderaan, you should get a final mission that concludes the first chapter.
2. As long as they're on the same server, your toons will get added to your Legacy when it eventually is created.
3. Shift-left-click the mission and it'll show up in the chat. You can do this with items as well.
4. Did you use the search field? Because I always leave it open unless I'm looking for something very specific. Or have you narrowed down the level range? I leave that 1-50 as well. Other than that... pick "Prototype" or "Custom" or "Artifact" mostly for most items in the rarity option, "Legendary" will be very hard to find on the GTN. You might also want to keep the "useable by" to "all". I must admit, it still is somewhat sluggish, but it does work, as far as I know.