If it's not a scam, there is always the possibility that the car has either a salvage title or that it has really high mileage.

I'll agree with that. Here in the Anchorage area, you will see a lot of high mileage cars for sale. Because of the geography and such, there is a good sized commuter community about 50 miles north of the city. You will find lots of cars with high mileage because of this. We just saw a 2006 Honda Accord for sale with over 170,000 miles on it.

You will also see the salvaged title issue on Craigslist ads. Most seems to be pretty upfront in their ads, however some try to infer that the salvaged title was due to some really minor accident.