Except if you were to place an ad in a paper it would be for a fixed fee, eg selling a car.
No sliding scale and then further charges on top if / when you sell the item.
Perhaps it's me but I think most people would consider a fixed price fairer, even if based on the final sale value. eg £1-£99 = cost X, £100-£499=cost Y, £500 - £999 etc
Also imo