You volley, Meaning buying allies back and forth. Each time an allie gets bought back from you, you make a small sum in return. Im not sure the exact amount but I THINK its about (5%?). You also need allies for your growth. I would refer you to check one of the guides but I cant im on phone so good luck Happy volleying
I think jrod is referring to the bigger players buying 100s of allies and all of them getting hired for a gain each.
I've made billions buying and selling allies (playing the ally market). I buy at a specific price point, making certain the ally is underpriced (stats warrant a higher price), then let the ally get bought without buying back. I have turned over an entire slate of allies as many as three or four times a day. The ally market will heat up and slow down over the course of a week, but I have never had to keep an ally longer than a couple days. Good luck!