worst thing you can do is leave it until the last minute. We had our cars on the market well before we were due to leave and went through the usual low offers but sold one to a genuine guy (Canadian) looking for a car for his wife. I eventully sold mine back to Honda, who had maintained it and knew the true value. I know they made a profit when they re-sold but so what. I was happy with the price.
Beware of those agreeing to buy the car, bailing out at the last minute and then trying to hold you to ransom. That has happened on more than a few occasions.