Quality gear. These can be greens, rares, whatever. However, always think, "Who needs this?" Items that have stats like Stamina are typically useful to all players. Items with stats like Spirit are only useful to a subset. Items that combine two highly sought-after stats, like Stamina and Intelligence (which all casters need) will sell for more than items that combine two stats like Agility and Spirit (which practically no class needs).
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.
2. Once you have an interested buyer, stay firm in your price. This does not mean to never make them feel like they are getting a bargain. You can subtract 5% off the price and not be hurt as long as you still make a profit! Remember this, as that 5% may not be a big deal to you, but the buyer will feel special and might buy the item for the slightly reduced price. I can not emphasize enough that this is not the same as undercutting! Do not drop every good every time or you develop a reputation as a soft seller and people take more liberties in their dealings with you. If you give someone a constant 5% discount, then they begin to want 10% and then 15% until you no longer make a profit. Discounts are used when you feel that the seller may be pulling away and you really want to just profit and move on!