Quote Originally Posted by Anova View Post
The marketboards require you to complete the Lv.17 main scenario quest, Scions of the Seventh Dawn. Doing so unlocks the retainer feature (which provides extra storage slots and lets you buy/sell materials on the marketboard).
They provide extra storage slots and let you sell materials on the marketboard. You can always buy there, regardless of whether you have retainers or not. I still recommend getting a couple retainers before getting far into crafting, but that's more because of the extra storage they provide. Crafting (especially if you're pursuing multiple crafting classes, which is the generally preferred way) involves a lot of materials and ingredients. Your own personal inventory space might get you through your very first tries at crafting stuff, but you'll quickly run out of room before you level your crafting very far.

Especially if you're starting your crafting relatively early in the game, you should pick up the gathering classes as well (or at least miner and botanist). A player who gets to endgame before going back and starting crafting may be rich enough to afford buying all their ingredients, but if you craft early enough to make your own leveling gear, you probably won't have enough cash at that point in the game to just buy everything. If you gather all your ingredients yourself, it's free (though obviously more time-consuming). You'll probably want a mix of gathering most of your ingredients, but occasionally buying a few of the cheaper ones.