We all know how many of these artists (Beyonce, Justin Bieber, etc) sell out within minutes.

They (especially Beyonce) get their team to buy the tickets they have on sale and re-sell them at a huge mark-up.

So for a venue of 15,000 audience, 3,000 would actually be up for sale; the rest will be bought up by Beyonce's team and hawkers so the can sell them at a premium.