What Makes World of Warcraft Akin to eBay?

Just what does it mean for something to be a virtual economy? This is a tough question, mostly because it's hard to realize what's necessary about the wide variety of different kinds of virtual economies out there. A few out of the many kinds are listed below; I hope you acknowledge just how distinct one kind of virtual economy can be as compared to another:

MMORPG's: examples include World of Warcraft, Everquest, Lord of the Rings Online

MMORPG's are the largest of virtual economies, but they also contain probably the most economically irrelevant aspects to them, such as fantasy and RPG-like experiences and player-vs-player combat. I'm just pointing out that WoW isn't characterized entirely by it's being a virtual economy; rather it is the case that WoW characterizes, among other things, a prototypical supply/demand virtual economy

"Life-simulation games" (this type courtesy of the rather incomplete Wikipedia entry on "virtual economies" available here)

Life-simulation games are virtual but probably in a more limited sense than the above. For instance, whereas the currency in MMORPG's is almost always worth something in 'real' money, the same may not hold true for Life-simulation games...at least not to significant extent. Examples include The Sims 2 and Second Life. Obviously the latter DOES have 'real' monetary value. Note Linden's "hands-off"--and more correctly, "motivating" mentality with regard to the selling of in-game currency.

Internet browser applications

Web browser games like utopia feature some of the properties of a virtual economy, but again, it's not necessary that there be a 'real' value in these games. You won't find many users buying and selling Utopia accounts and you certainly don't hear of someone exploiting it for real-world profit. That said, there are cases of Web browser applications earning real-world profit for its users (see my review of Weblo.com)

Internet auction sites/services: Ebay/Ubid.com

I consider auction websites to be a subset of the above category. However, given the relative distinctness of Ebay next to Utopia, I feel compelled to mark this category as its own. Simply put, these are services characterized by their virtual presence. It says nothing about what is being bought or sold, and more often than not Ebay users are probably buying 'real life' goods/services (although some virtual property is sold--e.g. WoW accounts)

What makes each of these categories categories of virtual economies?

I'm not sure. I think the best bet is to assess each one individually rather than attempt to sweep them all under one categorical picture of what it means for something to be a virtual economy.

Variance amidst members of the same category

Disregarding for a minute the variance among types of virtual economies, note that in some instances, members of the category of MMORPG must be distinguished based on important parameters such as "MMORPG publishing company's stance towards the buying/selling of in-game gold"

This is a huge one that divides World of Warcraft from something like Fury, a new MMORPG (but focused exclusively on player-vs-player combat) in which the developers "apparently" try to sell you in-game gold.