Skinning is both highly profitable and convenient, in that you will be skinning the monsters that you're already killing as you level. A skinner starts by collecting relatively worthless ruined leather scraps, but soon moves on to light leather, which can be sold for a good profit. An excellent way to farm leather is to skin the kills of other players, especially if you are following along in the wake of a group. However, do wait to start skinning until it's clear that the other player has abandoned the kill. Don't assume that the other player doesn't skin just because they are not a leather-wearer, for example. Note that your chances of getting a better grade of leather increases with your experience; it is possible, though rare, to get light leather from rabbits. Higher grades of leather yield higher profits. A skinner/miner has potent earning power through the auction house, but often runs out of inventory space.
I recommend not counting on transmog sells for your monthly gametime. Its a great market, but also pretty random. You have to have the right items posted at the right moment if a player needs a certain item. If this is not the case, of course you will not sell anything to him. There are people making millions of golds a month with transmog. But they are very professional at it and it really takes a lot of time.