The Material NPC in the housing district doesn't sell anything that the guilds sell. There's mid-level mats sold there that are unique to those NPCs.

That said, the above poster is correct, Material vendors in your house sell stuff from all the guilds so your members can level any crafting class up to level 20 or so without needing to leave the house if they so desire and it gives you a place to get random low-level materials without much hassle.

The Junkmonger NPC sells Dyes and Garden plots.