@tomdc You're missing the point of the trick.
The entire crux of the trick is that the merchant couldn't have hired 30 mercenaries in the first place; the merchant didn't have the funding initially. So there would have been no way for the merchant to have transported 3x as much good as normal without Adis' deal because he wouldn't have had the guards to do so in the first place.
It has nothing to do with the quality of the mercenaries directly; it has to do with the idea that the merchant somehow transported 3x as many goods as he normally could have, even on the same expenses, and that the merchant can't just say "The Blue Wolves did it for 3x as cheap" due to the conditions in the contract.
It's like saying "You spend 10 dollars to make 50 dollars profit. I spend 10 dollars to make 100 dollars profit. Our process was the exact same, except I used a different dealer than you. Therefore, my dealer must be good."
(Although that said: I'm not sure why the merchants wouldn't immediately just assume "Oh, the Blue Wolves gave them a good deal" or "The Blue Wolves were just desperate for a job." Maybe either way, it's profitable to the Blue Wolves for getting a good reputation.)