I think what jhpriestly is saying is: As the market gets efficient, the price of renting mining capacity will approach the profits earned from transaction fees. So renting mining capacity will almost pay for itself, i.e. is almost free. But then you might as well rent a lot of it, like 51%, because it's allowing you to attack the chain almost for free.
I've heard an opinion (but I'm not certain) that mined coins are considered less traceable than purchased coins.
For any purchase, there's a trail that leads to you through however you paid for it; for mining, the mined coins are totally disconnected from the hardware that mined the block and how you bought it.
Well then the buyer and seller would race to gain 51% of mining power (as its free) which will make it more expensive because of the constant biding war.
