Doom972 said:
I don't see how a random number generated on a server has a value outside of that virtual world, in which it has a use.
Also, what's the point of buying the best items? Once you get the best items, you have nothing more to do in the game.
It may be better to keep your point of view, maybe you even know the answers to your questions, but i'll try to put it together and clarify a bit.
On your first point: At first, it may only be a random generated number, but ingame, this number becomes an item, which has clear ingame use, in some way a resource and thereby becomes desireable by players, of whom in return, given the opportunity, some are willing to pay for to recieve, just as you are willing to pay to recieve a copy of a game. This may not seem an obvious simile, considering the game had production costs and such, but it if you think it through, it is, considering to receife the item, due to the mechanics of the game, there was effort needed, just as were when making the game. Thereby the random number-> item gets value. Now second, you question why anyone would want to buy the best item, considering this is kind of the goal of this game. Now this is rather simple psychology. As the game sets the goal of always becoming stronger, every step potentially getting you closer to that goal will probably make you feel like you achieved something, give you satisfaction, thereby every single step becomes desirable and for some this includes buying better items. Considering you can never be sure you have the best items ever and the game lets you experience non-linear satisfaction, it stays desirable (Skinner box experiment proved it). It may even be enough to just have an item, as is enriches their "ingame-wealth", which may be desirable for some by itself.
And if you still question how this virtual world resembles to our "real world", think about it, cause this is actually part of our current economy, considering our (the societies) ideology of money and wealth in numbers already surpassed the actual values of whatever you might consider that matters, be it to provide basic needs, or happiness, or whatever. Not everyone may agree with me, some claim that the bigger financial and bank institutes still monitor the currency and transition from resources to money, but i think, that the biggest part of our economy consists of actually trading with virtual goods, shoving numbers around, which in reality represent nothing, neither anything that matters, nor of matter.
The only real effect of this is an increasing social inequality, what isn't only effecting statistical numbers, but real peoples lifes.
And here we came from D3 Items to economy in a nutshell.