What would you charge?

just wanted some feedback to what you folks would say is reasonable commission for selling someone else's car.
Got quite few people wanting me to sell their motors but after headache of that last sale I realised it's not that easy.

My chraging figures have been all over but I want to fix one rate maybe depending on value of car.
Under £1000-£rate?
Under £1000-£3000£Rate?
Or is % on sale value better

Let me know what you think thanks