What is a Virtual Goods Store?

A point of contention with some EVE players is the difference between items that are spawned into the game by players spending time in the game, versus items that are spawned into the game by players spending money at the game. Virtual Goods Stores are the avenue through which items are spawned into the game by spending real money, or a real money equivalent.

I'm stuck.

I'm suffering a little from writer's block. I wanted to write about the economics behind arbitrage hauling and speculative hauling, as part of my attempt at discussing "balancing the economy".

If anyone can point me at suitable reading material to discuss the economic principles behind trade overall, and arbitrage hauling in particular, I'd love to hear from you!