My last two car purchases were out-of-state; one private party and one dealer. A few thoughts I'll add to what's already been said...
- Skip the emails and private messages in the forums and talk to the seller by phone to get your questions answered. You'll get a better sense for the type of person you're dealing with and its no secret that something is lost in text communication. The majority of people I've spoken with and met have been normal, good people just trying to sell their car. If you talk to someone and they have an attitude or make you uncomfortable for any reason, just thank them and move on to the next car.
- If practical, it can help to take an observant friend or family member along who may notice things that you miss as you try to look the whole car over, or during the test drive. If you're uneasy meeting up with the seller, taking another person along gives you an added measure of security as well.
- I agree with the advice that you should have the car checked out. Personally, I think I'd almost rather risk wasting a trip so I can be present for the pre-purchase inspection to ask questions of the shop and represent my own interests, but I can see the benefits of having an inspection done ahead of time as long as you trust the owner and shop to be fair with you. I guess that one comes down to the individual situation and personal preference.
- As far as your negotiating power goes, it's possible the seller will have an attitude like O-cha talked about where they figure that you've made the trip and they don't have to bend on the price because you won't want to leave without the car. It can also work the other way around if the seller is at all motivated, though. The fact that you've traveled a distance to see the car means you're not all that likely to go home, think it over, and return as a local buyer might. Faced with the possibility of you heading back home without the car, the seller may be willing to bend a little more on the price to get the car sold that day. How much attention the car has received locally and how motivated the seller is will probably be the determining factors there.
- Because sellers sometimes overstate the condition of their cars, if you're traveling a distance to see one, it never hurts to line up a couple others in the area or along the way so it's not a wasted trip if the original car turns out to be a letdown.