Don't do it. They're the biggest highway robbers out there!
You may see 'some' increase in replies and a slight up lift in sales during a busy period, but it will only level out over time to show you're only covering their extortionate prices at best. During quiet periods that cost will suck your blood dry.
I was with them from 'day one' of AT Digital (it was a pound a car and they begged us to come on board) and have watched them turn into the greedy vampires they are now.
If you go to their site and click on 'trade adverts' and then 'private ads' you'll see that the trade is 80 percent of their business, but they couldn't give two hoots about us.
The pricing is insane and value for money is appalling.
I noticed very little impact when I finally kicked them in to touch. The punters search for cars differently nowadays and most simply do a google search and, as long as you're with a few of the other companies, they will find you.
I use some of the £3000 a month I've saved on a full page local press ad once a month. I also advertise with:
ebay motors pro (ensure you enable 'best offer option', motors, car gurus and Exchange and Mart. My website is hosted by the brilliant Click Dealer.
Times are are hard at the moment and we're in for a tough ride. Autotrader are dinosaurs and you can do without them.