Obviously you don't want to spend hours sifting through all that data and figuring out which sites are worthwhile to list on. Luckily, there's only a few that are really important.
The two largest are Cars.com and AutoTrader.com.
Cars.com syndicates their listings to over 175 newspapers and partners such as Yahoo Autos. They have about 3 million used cars for sale at any given time.
Their premium listing is $55 and comes with a 90-day money back guarantee along with a free CARFAX report (worth $34.99). They also offer a $15 basic ad listing which will show your ad for 21 days, but with no money back guarantee or CARFAX report.
AutoTrader.com offers a similar package. A $60 listing gets you a money-back guarantee while their less-expensive $20 listing stays active for 3 weeks - but with no money-back guarantee.
Cars.com and AutoTrader offer a huge database of used car listings, but most are actually dealer listings.
The largest for-sale-by-owner site is Craigslist. The listings here can be very un-organized, but it's definitely worth including your vehicle - and the best part: it's Free!
You'll also want to include your vehicle in eBay Motors Local Classifieds section. They will display your listing to anyone that searches for your car within a 200 mile radius of your location. The service is free and even comes with a limited CARFAX report worth $19.95.
By listing your used vehicle on these four sites, you should have all your bases covered.