The buyback prices in handbook.json are way off so the easiest way to get the traders to buy everything back at lower cost is to edit each traders base.json and add a discount for each. I have set mine to 50, which means they will buy everything back at 50% of the price in handbook.json.
Each traders folder can be found in \Aki_Data\Server\database\traders
and handbook.json can be found at \Aki_Data\Server\database\templates
For really big ticket items like bitcoin, I recommend editing handbook.json directly to ensure you are getting the right price from Therapist.