1. There is no real system in place *yet*, but I understand there are plans to incorporate that still. As of right now, everyone goes to the hub zones to do their auctioning, much like East Commonlands used to be used in Everquest. The alliance players all go to Stormwind Keep. I haven't played horde, but I think they go to the Crossroads in the Barrens.
2. Crafting is a little tricky to explain. First you have to buy the basic skill from a trainer at the cost of "skill points" which you get from exp gained from raw mob slaying. (It's used for a couple other things, but not in bulk like tradeskills need right now.)
All tradeskills that produce things generally have a gathering ability. Leatherworkers really need to be, or have a good friend that does skinning. Herbalism and Mining are other gathering skills.
Once you have the materials and the basic skill to do, say, alchemy, you open a menu where you can brew the alchemy recipes you already know. The menu will list the recipes, along with how trivial they are, and how many you have the materials to make. Combines never fail. The trivial level is only for determining skill-up rate. Additional recipes can be bought from the skill's trainer for money, however some are drop only and you have to acquire from mobs. They all have a required skill level to "scribe".
If you have the ability to make an item, you ask the game to create it, a small bar appears at the bottom of your screen as you complete the item, and reaches completion 4-5 seconds later.
The interface is clean, and simple to use. I find tradeskilling much more enjoyable in WoW than Everquest.
There are a few downsides though. Gathering herbs for your alchemy can be a full-time job, and when you reach certain plateaus of skill in a trade, you need to spend more skill points to advance from Apprentice level to Journeyman to Master, and there will be more levels at release. Players argue about how useful tradeskilling is right now; with the latest push, the crafted items have apparently taken a hit in relative power to the items mobs drop. And sometimes you reach a skill level where there is simply nothing you can do to gain skill-ups without doing one of two combines that both require the same resource, assuming you haven't found any non-vendor recipes.
The whole system is still under some measure of revision, ofc, but I find it to be much more pleasurable than Everquest, where I adopted the slogan "Friends don't let friends do tradeskills."
3. You can do as many crafts as you have skill points to pay for. However, while an initial cost of 5-15 points is not bad, advancing to journeyman costs 50-100 points (per skill), and the third level is even higher. Chances are, you will probably not want to focus on more than two skills, one being a resource gathering skill. When you get higher in level, the skill points come much more quickly, but you have to go to the newbie lands to practice your skill on basic stuff before you can, say, mine the ore deposits in the higher level areas. So while you can do all you want later, it's good to get a start on *a* skill early on if you intend to.
4. Selling to a vendor, crafting isn't profitable. However, some player-bought goods have decent earning potential. I've never been a good entrepreneur, so I'm not the best to answer that question, but some people do make money enchanting people's cloaks or selling engineered helmets and tailored backpacks.
Only the exceptions can be exceptional.