[scootr]

Patience is the key to getting a good deal and a clean car. Carfax is your friend. And so are vin code decoder sites. A lot of times dealers don't know what they are selling. I was looking for a manual transmission. You won't believe how many DCTs are listed as manuals !?
You can easily become expert on the value and the options that were available for the model you are searching for.
Ask for a walk around video - even if it is a cel phone video a salesperson sends to your phone. You can pause and enlarge any section or thing you want to take a better look at. This is an awesome alternative to seeing the car in person.
I think it is safer to buy from major dealers rather than small 'dealers'. These small used car dealers should be avoided, they usually sell seconds. And as I found they can be criminally misleading. That's something you will not find at a manufactures dealership even if you are buying an off brand trade in like I did.
I think the new car dealers are a much safer place to buy online. They have a reputation to uphold.
I've bought 2 cars this way now. Both are great cars that were not available locally. When you are negotiating with the seller be sure to tell them you will be posting a 5 star rating on Yelp after the sale. They know this goes a long way towards their future online sales credibility and you should be reading reviews on any place you are planning to buy online from.
Do your homework. Learn your car inside and out. Search nationally using every autotrading car.com type site and the bmw cpo locator tool