During this year I listed 3 cars on Autotrader (various times of year) and its the most expensive option 6 weeks for a figure around £60 each advert. Yes one car did sell within one day (it was priced right) and the other two cars which were the best examples available at a good price had no interest for 4-5 weeks. I found with Autotrader you get LOTS of fake emails which are generic asking to ship the car etc (which I know are fake) and fake texts to your mobile. Even gets to the stage when Autotrader admin send you an email making you beware of a false user who has just emailed you!
Car and Classic is also a great site to even sell a modern classic if its rare and has something about it.
With ebay I found the best way to advertise the car is through a Classified Advert without an offer option, this way buyer can come and look whilst the advert is still live online. The first time round the offer option was put on and I accepted the offer, the buyer came and choose that this model of car is not for him (nothing against my car at the time) so now I was faced with ebay charging me to list that advert and final value fee. I did re-list without the offer option and claimed my money back through ebay which they did help with.
I find model specific forums hard to sell your car as everyone has one, but a brand generic forum might be better.
Never used gumtree/pistonheads so cant comment, my last resort would be to do a px with a dealer unless your after a very quick deal in which you will lose out on.
Hope this helps.