The surprise hit for me has been the Blazing Rapier. I literally cannot make enough of these, the vast majority never make it past the first posting. Obviously I don't flood the market, I only sell one at time (and wait until that sells before listing another). Even with my conservative listings I just can't keep these in stock for more than a couple of days. My biggest bottleneck is restocking the mats for it.
Another terrific seller is the Imperial Plate set. As far as I'm concerned, I basically created this market on my server. The first week I was the only person posting items, but this week I'm seeing a few others jump into the game. Luckily they are sticking with the same general pricing that I've been using. The great part about this set is that you can set a significant markup on the items. The mats for each piece run about 20g and I am consistently selling them for 200g each. For me the best strategy is to post in full sets, I notice the sales go more quickly when you have all six pieces ready to buy. The couple of times I've listed just one or two, they were generally ignored. And don't worry if another person posts a flood of onesies on the auction for cheap, just list the other pieces. People will still pay full price for the slots they need to get the look. Another set I've just started testing out is the Thorium gear. I've already sold a couple of sets, this may be worth trying as well.
By far the strongest markets are in plate gear, cloth gear and weaponry. So far I've sold 8 Blazing Rapiers, 4 Fel Iron Greatswords, 2 Phantom Blades, 6 Imperial Plate sets and 2 Fel Iron Plate sets. This is just with modest crafting and posting efforts, I spend maybe an hour a day across 5 different crafting toons.
My Tailor has also been getting some great action with transmog, the appetite for robes is practically insatiable. At first I focused on the Mooncloth Robe and I've sold a handful of them (the golden pearls are a pain to keep in stock). But after seeing how picked over the cloth listings have been every day, I expanded into Crimson Silk and Black Mageweave sets. Strangely, I see lots of folks posting the BM Robe but less action on the BM Vest (seriously, this is just sad). Speaking of sad - don't forget cloaks! I've already sold 3 Cloaks of the Black Void that were just languishing in the bank because they're an awesome solid black with gold trim. Perfect for a rogue or death knight going for that dark and broody with a touch of class look :)
The main disappointments I've had so far are with the epic Burning Crusade cloth sets and Leatheworking in general. My Tailor made a full set of Primal Mooncloth and the key pieces from the Frozen Shadoweave. It took me about a week to finally sell the Primal Mooncloth set, so these items are slow movers. Admittedly, I'm charging 2000g per piece, but it took me two months to collect all the dang primals, so I think that's reasonable :P With my Leatherworker, I made four sets of Black Dragonscale and I've only moved two of them. For such an iconic set it doesn't seem to get much action. My other leather items have been even harder to offload. On top of that, Leatherworking doesn't have much to offer in the way of eye-catching goods. So I've given up on transmog leather and I'm focusing on leather PVP gear instead.
In addition to crafting, I'm also flipping. When it comes to cloth robes I'll buy anything under 10g that has nice artwork. Then I relist them for 99 gold each, occasionally I drop it to 49g for a really low-end item. For plate, I have a list of about a dozen sets that I pick up any piece I see at a low price. I've got MogIt installed, so I can also browse the listings and snatch anything I spot with appealing artwork. MogIt creates a pop-up image on every item you mouse over, so it's really quick to "window shop". This gives you a serious advantage over other shoppers since they aren't likely to
The really fun part of selling gear for transmog is when you browse the listings and half of the lowbie gear for sale is yours. I'm having a blast dominating this market :) And this is one of the few times when my packrat ways have paid off. I've been collecting gear since the transmog feature was announced. Every time I farmed a dungeon to get classic armor for my girls, I saved the pretty items instead of vendoring them. Because my rogue had to run dozens of BC dungeons to collect her Outland Bloodfang set, my banks and mailboxes were overflowing with gear by the time the patch dropped.
By far the most awesome part of transmog is when you see another player walking around in the gear you made. I've already seen a couple of priests in my Mooncloth Robes. Now maybe they made that robe themselves, but I'm the only seller listed on TUJ so I'll hog the credit :) Go forth people and wear my wares!