Top tip for using autotrader:
If you create the advert and get through to the payment screen, then leave the page without paying and wait a day or two, they'll call you up to ask you why you didn't finish creating the advert and offer help. If you moan at them about the price, they'll usually give you a 10-20% discount, or offer to put it in the magazine for the same price as putting it online. Don't feel too bad about doing that, it costs them practically nothing to put it on their website, and I expect their magazine is mostly paid for by business advertisements, so a discount is just marginally eating into their huge markup.
Similarly, if you fail to sell, they'll call you up and ask if you want to re-run the advert. A good moan about the price and how few responses you got will net another discount/upgrade.
I've found that you can get better responses through company mailing lists. If you work at a big corporation there's often a mailing list for buying/selling things, and putting the car through that first might get you results.