What's the #1 money making item a 350+ tailor can make and sell at the AH?

The steadiest moneymaker for Tailors are the 16 slot bags.

Best thing is to make Netherweave and Imbued Netherweave bags.  They sell great!

I found that the 18 slot bags dont give that much money. People still make them to level tailoring and it seems that they are willing to sell them for 35-40g but the mat cost to make them is slightly more than that. The 16 slot bags though on the other hand can make you some money.

I buy my netherweave for 3g a stack add in the runethread and that puts the mat cost at about 4g each for the Netherweave bags. You can easily sell these for 5.5 but a lot of the times I get close to 8g per. So netherweave bags = 1.5-4g profit each if you buy the mats. Not much for one but it might by one of the only things that you can make gold from. 

I just farm BT all day I get loads of Netherweave, and a tone of Arcane Tomes and Sunfury signets also. after about 6 hours of farming I'd usually have 4 bags full of netherweave which I turn into bolts then fill the bags up again, I always keep about 200 Rune Threads on me so I can make the bags and sell them at 6 gold each, I usually sell about 15 to 30 bags a day... and I usually make around 500g a day from the arcane tomes and sunfury signets.

The best way to make money I have found is to make items and the DE them you can make a brown linen robe and DE it into strange dust and lesser magic essence, at the mats price on my server you can easily double your money doing this, also make a white bandit mask that DE's into Vision Dust and lesser netrher ess or small radiant shards, this is a really good because the mats used to make it is very minimal and the amount of mats you get when you DE is high, if you cant make gold doing this with just these two items you need to change server.

WoW 2.3 currently LPS's have inflated to 30-45g on my server (horde side) [gilneas] so if you make some of the 65+ netherweave blue boe's you can d/e them for alot more than the mats witch is some dust and spider silk. im sure coming 2.4 this will change and might not be so usefull.