Obviously, you have issue sin your transmission, just by your statement.
Did you know what you were buying into? When you buy a car at auction, who knows what you are really getting.
Did you actually look up those codes:
Code P0715
The P0715 code indicates a malfunction in the Input/Turbine Speed Sensor circuit, which can lead to issues like abnormal transmission operation and decreased fuel economy. It is important to diagnose the underlying cause, which may include a faulty sensor, wiring issues, or problems with the torque converter.
Code P0740
The P0740 code indicates a "Torque Converter Clutch Circuit Malfunction," which can cause shifting issues, stalling, or reduced fuel economy. It's often due to low or contaminated transmission fluid, a faulty solenoid, or wiring problems.
Find yourself a good transmission tech. This looks expensive