This is really quite simple. Murphy was the wealthy collector and victim. He wanted to acquire rare firearms. He was not an expert, however, so he hired Ellis to help him. Ellis's job was to help locate rare and valuable firearms for Murphy, and help him decide how much to pay for them. In other words, Ellis was supposed to help Murphy determine what a reasonable price was. Ellis got a commission on the purchase price -- roughly 10%.
Thus, Ellis and Murphy go to a show together. Ellis shows Murphy a really nice gun and says, "the dealer has it marked $5,000 but you should be able to get it for $4,000." Murphy buys it for $4,000 and pays a $400 (10%) commission to Ellis. Everyone is happy.
The problem here is that Ellis conspired with Zomber to cheat Murphy. First, you need to understand that Murphy was not buying commodity guns. The stuff he was buying was so rare it would not show up at a gun show. He was buying from other wealthy collectors. Ellis would locate an item that Murphy might be interested in. As these are by definition unique items, there is no "market" to refer to for a price. The value is whatever someone is willing and able to pay, right now. If Murphy is the only potential buyer, the value might not be that high. The owner of the Walker Colts, for instance, may want to sell them, but if no one is willing to offer more than $1 million, then $1 million is what they are "worth." Murphy could have bought them for $1 million, and Ellis would have gotten a $100,000 commission.
In this case, though, Ellis got his friend Zomber to pose as a disinterested buyer for the purpose of driving the price up. Zomber expressed (false) interest and a willingness to pay a lot of money for guns Murphy was also trying to buy. As a result, the price Murphy had to pay went up, just as if they were at an auction and Zomber was acting as a shill bidder. Ellis benefitted from driving the price up because he earned higher commissions, which he then paid a part of to Zomber.
The point is that Murphy ended up paying a lot more than he would have had to pay if Ellis and Zomber hadn't scammed him, AND he ended up paying Ellis more in commissions than he had rightfully earned.