In terms of other players, they buy accounts and gold because they don't want to put in the time to get it themselves. They want to play at level 60 but they don't want to spend 3 months playing whenever they're off work to get there. They'd rather just buy a level 60 and play occasionally when they want to. Same goes for buying gold. Getting gold in the game isn't the fun part it's really boring so if someone just buys the gold they get all the items without any of the work.
Like if you're working 40 hours a week making $10 an hour, you could buy 100 gold for one hour of work. However, in the game it could take you 4 or 5 hours to make 100 gold. Some people know secrets to making gold faster and some people are horrible at it. Easier to work 1 hour in reality for 100 gold than 5 hours in a game for 100 gold, if you suck at making gold.
The reason Chinese farmers would rather lease out an account instead of leveling their own is that they'd rather spend all of their time farming gold rather than spending 1/2 a month leveling up a character. Also since they are in China they're actually restricted from playing on North American servers. You're providing the North American account and it's been played from a North American IP address. They require you to be the original owner of the account because Blizzard sometimes bans accounts that change IPs from USA to China. If you're the original owner you can call Blizzard and tell them your account was stolen or whatever and get it unbanned.
Also I'm not sure if this happens in WoW at all but in other games I've played people who are cheating to farm gold and such who have obscene amounts of gold can manipulate the economy and almost make buying gold required.
If you have unlimited money in the game you just buy all of the nice items that are for sell and then resell them at the price you want. Let's say this sword is selling for 100g and you buy it and sell it for 300g. Every time someone wants to sell one of these below 300g you buy it and resell it for 300g.
That sword might actually only be worth 100g but you've made it impossible to find for that price. You do this with every weapon and basically make it impossible to find cheap weapons anymore. The amount of money people are making in the game hasn't changed though. Someone who would have payed 100g for that sword won't be able to pay 300g, they can't afford it.
Guess how they can afford it though? Buying gold. Then you sell gold in allotments of 300g on the server you're doing this on. Makes it so tempting
Eventually you don't even have to control the economy anymore. People will realize that you'll buy this sword if they sell it for less than 300g but then they're realize you're just reselling it and they'll start selling it themselves for 300g and you've just manipulated the price of that object for a long time. Basically it is worth 300g now because people will buy gold online to pay for it since that is the only way to get it.
