The Veteran - Most buyers I think will be ambitious experienced players wanting to be a little bit stronger. Someone like me in fact - if I spot a good deal on a +magic find item I don't mind dropping a few pounds on it. Maybe it'll pay itself off. Some of the systems in the game seem very cleverly designed to get people to spend just a little. For example the skill planner I've been playing with recently uses level 7 runes which are ilvl 58 and presumably only drop in Inferno. Some of the builds I made need the full bonus of the rune to make the build work, my Teleport build would be much less good if a cheap rune was in the Teleport spell and that meant you had too short a window to find your next destination. So you can build something on the planner, get all excited, realise you can't make it work with what you're finding and be drawn to the RMAH to make your build work. People will get sucked in.
The Noobs - Tobold thinks the market will be driven by people who are not very good but want to disguise that by paying for success. He's right that it's a factor, I'm not sure this will be the biggest driver of the economy though. They can pay huge amounts. One DDO player spent $200 on mana pots from the cash shop in the first month of free-to-play. He could have kept them healed using wands bought from vendors cheaply for in-game gold. He didn't know about that and didn't want to tell the others he couldn't keep on healing them.
The Traders - some people will buy items they have no intention of using simply because they think they can sell at a higher price. Sometimes they will be right and make money, sometimes they'll get it wrong and lose out.
The Narcissists - some people will buy items because of the appearance. Dyes, flashy spell effects and cool looking armour will have value to them.
The Big Spender - anecdotes abound of That Guy. You know of him, you've heard about him. He's the guy who spent $6m on his house in planet Calypso; the guy who bought over $100k of plex in Eve or the celebrity whose wife owns a Spectral Tiger in WoW. Of course what these people are really buying is celebrity, the fact that they value owning something that other people can't afford is a phenomenon that gave rise to the unfortunate cash shop monocle in Eve. Count on hearing stories of spectacular extravagance in D3 (because the whole point of spending 1000 times more than anyone else is to have people notice "wow, that guy spent 1000 times more than me").
The pvper - competitive pvp generally forces people to min/max. While there is some scope for coming up with an original and powerful build early on the scene quickly becomes a matter of using cookie cutter builds with specific optimal gear. Anything on the gear list of the most successful cookie cutters will be in high demand.
The regular players - by far the biggest RMAH input will be from normal unremarkable players turning surplus loot into cash. Most of the time these people will never use the AH either to buy or sell but occasionally one of them will find an item that people tell him is worth a decent sum. And he doesn't want to use it. Not individually very economically busy but so many of them that they will dominate the market supply.
The amateur farmer - someone who farms and sells it because it's a minigame. The profits are a way of keeping score. This will be a popular playstyle and a significant source of supply. This is where I'd classify myself.
The trader - buying from other players and re-selling at a profit. I think this will start off quite popular but will burn a lot of people. There are players who think they're incredibly shrewd based on WoW or Eve who are going to get burned when they start playing for real money. It's like someone who usually beats the family at cards over Christmas thinking he'll be great at Poker in the casino. Some people will make money of course and quite possibly a handful of people will make a lot of money.
The Chinese gold farmer (blue collar) - traditionally MMO gold farming has been a low skill low status job in the Asian economies where this has become an industry. The worker moves his character to a zone with non-elite monsters intended for soloing, presses a couple of attacks and collects loot. For hours. At the end of his shift he sells off his loot for anything he can get and a replacement worker starts a shift (selling off anything his colleague failed to liquidate). I think this business model will fail in Diablo 3. Not completely, these guys can farm Nightmare or early Hell but late Hell and Inferno will be too hard for them. The way they share the characters in shifts means workers will sell off all of the character's gear at the end of their shifts which will really mess up the character's ability to farm. The tendency to solo hurts them and the tendency to view this as skillless menial work. There simply won't be much money in mindless farming. We may see these guys turn up in random pugs as magic find leeches.
The Chinese gold farmer (white collar) - currently white collar work in the industry includes things like customer service where the worker has to be a bit more switched on and clued up. (Especially if the exchange of goods is accompanied by social engineering aimed at getting the client's password). Diablo 3 will surely see a new era of professionalism in gold farming where gold farming teams start recruiting good players, where people have sole access to a character, where they keep some of their good gear, and where they play in synergistic teams. I don't think many such teams will be ready to go on Day One but I do think we will see a transition to this format during 2012 and 2013. A feature of this business will be that they trade through Blizzard's system, effectively they're real players who just farm all the time and don't do anything outside the EULA nor behave different from most other players.
The Chinese scammer - scamming, phishing and hacking is the fastest growing part of the RMT industry. This business model is dishonest all the way through. Typically they pay for an account with a stolen credit card number, offer item selling or power leveling that is mainly an attempt to get passwords, phish for passwords, then after a few months log onto their clients' accounts and strip them. These guys will sell D3 items cheaper than the RM auction house to get people to visit sites where they'll use trojans and keyloggers to steal player accounts. They'll get cleverer too so we'll start seeing informational websites and forums that look like they're done by an English-speaker which are actually attempts to steal people's information. Do be careful, I think this sector will get more sophisticated and more desperate because I think Blizzard's model marginalises them. A lot of people buy items in games, most of these will use the obvious in-game system. These businesses can't trade on that system if I'm right in assuming that Blizzard won't let you take real money out until your credit card has cleared. They have to lure people to third party sites.