If your phone has Auto-ISO setting, turn it off and put it on the lowest ISO setting (it will reduce the graininess), and take your pictures with lots of ambient lighting. If there are other preset settings, turn them off. That might help. There is no way an iPhone 4 should take such grainy unfocused pictures. My original iPhone can take better pictures and it doesn't even have autofocus!
Anyway, good luck with sale!