First off, glad to hear you was injured in any way!
Now to some more serious points...
Why was this motor handed straight over to you, the customer, without being run up and checked by the seller?
Why did YOU not check the correct closing of the throttle in your preflight checks?
Were you not instructed on how to 'emergency' stop the motor using choke, primer bulb (if fitted) to flood the engine?
The main point though, glad you came out wiser......