how can I search several cities or states at once?
Sure. When you plug in all your information for the car you are looking for, then your zip code. you will see a 'Radius' ..that's the distance you are willing to go if you want to look at a car, click the down arrow by Radius and "Unlimited" is coast-to-coast
Sounds easy but I can't find the screen that shows unlimited. They all say up to 200 miles and no more.
Ebay will show you coast to coast. Also, carmax. Craigslist will also though you have to select the cities individually