Think the auto trader online is free?
Loot is also a good one, what about your free local paper, they wont be free but they cost pence rather than pounds to advertise? Also might help with buyers as the car is more than likey to be local which will encourage people to come and see it.
On another note if you are going to put posters up in the car with a mob number please please use one of those pay as you go chips on a number you don't want to keep you'd be surprised how many wierdo's there are especially if they see a young girl driving the car,
In fact if you want a o2 one i got one you can have still sealed, gis a shout flower and good luck xx