gas tanks

i posted a thread here but know ansers. does any one know how to drain a gas tank without damageing it. i cant sypon it couse it has a sreen at the bottam of the filler tube .i tried to disconect the fuel filter line and pump it out with the electronic fuel pump but that did not work. i dont want to drop the tank.im selling the car and dont want to give up a full tank of gas please help.