I have sold through autotrader a few times and its ok to be honest, keep your price competitive compared to other options and make sure you take plenty of pictures, fresh MOT and make sure its all clean etc... I think a lot of the time appearance sells a lot more than ojective things such as mileage etc..
I sold my 2006 350z for £5600 (my silly pre-MMM car) I think at the time had about 4-5 viewings within the first week or so. I later sold a 2004 Fiat Panda, tried to go through the dealers but they were only offering £150 but I managed to sell it for £900 through Auto Trader, it didnt attract as many buyers to be fair but I listed it at £950 and it was 14 years old when I sold it, think I had 3 viewings and the last one bought it there and then. I sold my wifes Ford KA too, it was in a sorry state but priced it at £600 and we got loads of calls, again that was a 53 plate and around 14 years old when we sold it but it had scratches and poor body works.
I always paid for the unlimited ad so it stays up until you take it down, you can lower the price if you arent getting much interest. Remember to make sure that any people viewing the car and wants to test drive it shows proof of insurance so they can drive your car, I know many insurance policies offer this but good to make sure if you are selling yourself.