I absolutely agree with a lot of the stuff being said here. Selling is really an art AND a science if you want to get really good at it. I've never done this with RE, but have done sales gigs before and was pretty good at it. Not the best in the world, but solid. I was always kind of a soft sell kinda guy, and pretty good at reading people and adjusting my own behavior and words accordingly. Most of building good rapport comes down to being able to figure out the best approach to deal with a particular person. That will only come from studying sales techniques and experience if you're not a natural. Some people like direct conversation, some people don't. You need to learn how to tell the difference FAST. I was good at building rapport and getting people to open up, but will fully admit I wasn't always the strongest closer as I don't like pressure selling.
So definitely try to read people, and see what the best way to handle yourself is, because that will vary by person, especially if you're meeting in person. I would probably suggest starting out with non business-y, non sell-y kind of demeanor and words. Something more like "Every time I drive past the house at 123 A Street, I can't help but notice this property has seemingly been vacant for awhile, and was curious about the house. I really like the look of the place." As opposed to "I noticed this place was empty and would be interested in buying it." right off the bat.
After that, read their response, respond accordingly. Be direct if they are, more round about if they are, speak at a similar speed as them, folksy or proper fancy words to match etc. I have done this stuff on auto pilot even in non business settings for quite a few years at this point, but a lot of people don't realize how big a turn off it is for some good old boy to have somebody talking AT him sounding like a pretentious fancy man, or how turned off a snooty lawyer will be if you sound like a hick or a fast talking shiester from the Bronx in 1975! You don't want to come off as a fake or a parrot, but just subtly around the edges try to make the way you're speaking and acting a little closer to the way they are. It'll never hurt!
One thing I haven't seen anybody mention in this thread that seems obvious to me is: Just ask them things! For instance after choosing a proper way to intro, "Hey, so since this place has just been sitting for a long time, is there a particular reason you're still holding onto it versus selling it or renting it out yourself?"
In the sales I did one would often ask what people were looking to accomplish with the purchase. They'd usually have their mind set on a certain product, but sometimes it was out of reach financially, or even if not out of reach they weren't sure it was worth the cost... BUT if you could find out what they really wanted to accomplish, you might be able to suggest an alternative that suited their needs and was more in their budget.
In short: What is their motivation? What are their sticking points? What do they want to accomplish? What has held them back from selling or renting it in the past?
I'd be surprised if you didn't get pretty straight forward answers along the lines of things people have already said if you just asked people why they haven't sold or rented. "It was my parents house and I'm keeping it for sentimental reasons," "My brother and I jointly inherited it and can't come to an agreement on what to do," "I want to sell it but don't have the cash to get it in saleable condition," etc.
Once you know what's been holding them back, you can find a way to solve their problem! "Oh, so your brother is the pain in the butt one huh, give me his number and I'll see if we can work something out." "This was your parents house, well wouldn't it be nice to spruce it up a bit and pass it along to a nice young family so they could raise their children in the same house you grew up in." "You don't need to have the cash to rehab it yourself, I would be willing to buy it as is, or maybe we can even be partners on the project." Stuff like that. Once you know their sticking point or motivation for not selling you can think of a way of getting them unstuck or solving their problem. Probably best to rapport build before even getting into that stuff, but I think just asking could be a painfully obvious way to get to why they haven't sold already.
Nothing works on 100% of people, but I'd bet my behind if you just outright ask why they've held onto the property you'll get a lot of honest answers that give you somewhere to start in figuring out how to create a situation in which they might be willing to sell.