I've bought several cars online, through forums and eBay, and have sold on Craigslist and forums and online classifieds. I've had several good experiences, and walked away from many poor cars. What was likely my very best car ever was bought sight unseen, and driven home 900 miles. So there are good ones out there.

My rules:
Expect the worst when buying.
Research the price for a car that is nothing more than a drivable, restorable "core".
Research the price for a fully restored model with the same features.
Look at pictures, ask for more if you don't see what you are looking for, ask plenty of questions.
Pay only for the core, plus the good.
Calculate what it would cost to bring the "bad" to good, and add 50% of that to the core cost (what you can't see will probably be as much as what you can see). Pass on cars that will cost more to restore than what you can buy a finished car for.
If at all possible, spend plenty of time examining the prospective purchase and check the condition of fluids and tires.
Bring a knowledgeable but non-enthusiastic friend. A set of eyes not looking for just the good points is invaluable.
Never expect to drive your purchase home if more than 100 miles away. There are too many unknowns to be confident.
If driving, have a chase vehicle stocked with common supplies and spares and tools. Have a roadside assistance policy on your classic car insurance policy to be safe.
On occasion you will find a nearly flawless, exceptional car. More often, you will find something that is far below what the seller described. Be prepared to walk away empty-handed, but have cash in hand to buy the gems.