Eye up the area they live in and the person's situation if you can. Get chatting to them and see what info you can glean from them about how desperate they are for the money if it's a private sale and then however good the car is make an insulting offer. Very few people get genuinely offended and you can always work up to the price you are happy to pay for it and if you're inexperienced in buying make sure that all of the paperwork is present,and ideally take someone with you who is more experienced. Physical money is far more alluring to a seller than cheques, a lot of private people won't accept them anyway. Tempt them, and don't look in love with it, even if it's the car of your dreams, keep mentioning all it's underlying all it's faults however minor and look undecided at all times, you'd be amazed how low a price you can actually buy things for as most people advertise way above what theyre prepared to accept. I've traded cars for a living amongst other things for many years and the golden rule is start very very low:)
Good luck.