Since AH charges 10% fee to the seller when something sells, I know it's reasonable for people to offer less than the posting prices on AH. But I don't really know how much less would be called "fair" or "too cheap", because I suspect that we would try to get something for much less than 90% of the AH prices. I'm thinking something around 70%~80%, but most people are offering like 50% on the streets. It feels way too cheap to me, but so many people are doing it that I can't be sure my judgment is correct.
If I want to sell a lot of materials, what kind of price range should I look out for?
To maximize profit, you'd list items individually or in small groups in the AH over time, and only list new ones when the last small batch has already sold.
What you say about players trying to get an item for <90% its AH cost is very true; I've seen people trying to buy shards for 1cr (unsuccessfully), and others trying to buy a 90cr mat for 50cr (successfully). Since there's a much larger available market in trade chat, and no waiting to see if your mat fails to sell (usually if you undersell the other guy by a few crowns it'll sell), if you want to get rid of all your mats at once, you should advertise in trade chat or just /shout in front of the AH. Unfortunately, this will get you a pretty crappy deal, since not only are you selling outside the AH, you are also selling in bulk. If you need a few crowns fast, go right ahead, but if you wait longer and sell slower, you'll cut a much larger profit.