You can sometimes find great deals in the private sale listing. With some patience and time, online you can find the good deals. Many sellers online are doing it because they are under the gun and need the cash fast.
Often, private owners are not as aggressive when it comes to talking about numbers and the process. Private owners will allow you to take things at your own pace. Most car salesmen will try and move you to make a choice as soon as they can. This unneeded pressure can be avoided by finding a reliable salesmen or staying in the private market.