This was an example of an abuse of power and insider trading, where Trisha used knowledge and trust against Kerry to make more money. Kerry assumed Trisha was an honest horsewoman and trusted her assessment of Brumby's injury, but Trisha used that trust to convince Kerry to give her the horse for free under the pretext that she would be paying medical expenses on Brumby. Instead, Trisha sold Brumby without paying for the medical treatment she claims she thought she needed, allowing her to claim all of the profit of Brumby's sale.
Kerry is looking to sell Emma's (her daughter's) horse, Brumby. Emma also needs a trainer, so after searching Emma and Kerry decide to work with Trisha, a horse seller and trainer.
Kerry can only afford to pay for 2 months board at Trisha's, so Brumby has to be sold before then.
Trisha finds an amateur rider willing to try and buy Brumby in less than a month of Brumby boarding there. Brumby just needs to pass the vet check to be sold. Emma and Kerry start searching for a young horse to purchase with the money they will make off of Brumby's sale.
Unfortunately, Brumby does not pass the vet check so Kerry and Trisha discuss the plans on how to handle the situation. Trisha has Brumby checked by her own vet and tells Kerry treatment and stall rest were needed, which Kerry could not afford.
Kerry is told by Trisha that if she were to give Brumby to her that she would pay for all of Brumby's medical expenses and will sell her when she's recovered. Kerry agrees to this arrangement.
Kerry then discovers that Trisha has Brumby in training and sells her. When asked why she wasn't on stall rest, Trisha says the injury was old and wasn't affecting Brumby's health so she sold with a clean bill of health. Kerry received none of the profit because they had signed Brumby over to Trisha.