The scenes where Mattie is negotiating with Col. G. Stonehill are hilarious. More, she understands the volatile nature of prices when she comes back to buy one of the ponies she had sold back to him just that morning.
"I will take one of those ponies off your hands."
"What is your offer?"
"I will pay the market price. I believe you said the soap man offered you ten dollars a head."
"That is a lot price. You will recall that I paid you twenty dollars a head only this morning."
"That was the market price at that time."
Indeed, Stonehill has nothing to say to that. Prices are contingent to time and place. The time had changed, and the ponies had lost half their value. Having already accepted $10 a head, Stonehill was in no position to ask for the full $20 he had bought them from her for. We are also faced with an interesting situation: would those who would otherwise think of such negotiations made by Mattie as unethical (since she had sold them to him for $20 just hours before), think otherwise just because she was a teenage girl? If not, why not? When Mattie does it, it's funny; but if Stonehill had done it to Mattie, it's my guess that most people would have been outraged that he was "taking advantage" of her. But he would not have been taking advantage of her any more than she took advantage of him. The humor comes about because one does not expect a teenage girl to have a better understanding of commerce and price negotiation than a middle aged man.