Yeah. Simply pressing enter does not do it. <_< You have to join a channel to do so. Like, well, it was already mentioned.
Trade channel = /2 (said 3 before. But at least for my game, 3 = LocalDefense. Then again, this is inside the city).
General Channel = /1
Now, AFAIK, you can only join trade if you're in a city. Okay, that's probably wrong. It's the only time when you are automatically entered in that channel. Haven't actually tried entering it myself outside the city. :sweat:
And yes, for most items, do NOT buy it at AH. I mean, wool? Unless you're desperate to level up that First Aid skill, than simply grind for it (i.e. fight baddies that drop it at a decent drop rate).
Only a few times have I went and bought stuff at AH that I might have taken some time getting myself (mainly, ingredients to level up Jewelcrafting or Blacksmithing) and this was AFTER I have a ton of gold that I could 'waste' (after all, those are crafting professions. If, IRC, you only have gathering professions, than you shouldn't have to worry about getting mats).
Enchanting was the one I buy mats at AH more. But this is mainly because said skill requires more than just mining, skinning, or gathering. After all, to get the mats, you need to disenchant green/blue items. Not easy to come by (and if I do, I probably sell it at AH first anyways).