It's a free market for special item "drops" that you get in game, that have specific drop rates (rare items end up being worth something to collectors). It's no different than people paying for Diablo inventory back in the day, except that a) Valve controls it all so they get a cut of the market, and b) they are purely cosmetic, so people can't pay to be better, breaking the game mechanics.

It actually has very little to do with the question at hand because the wow factor in this case, the price, is user defined, and also doesn't really affect gameplay at all (AFAIK).