Many good replies here. I will just add, one thing I have done in the past is present two offers. This is not the first step, this is after you've built a rapport and they've expressed an interest in selling.
One is an all cash offer that I could actually afford with bank financing and still meet my investing objectives of 10% cash on cash return and $250 per door. The purchase price is obviously going to be pretty low, and probably much lower than the asking price.
The second offer is a seller financed offer that allows them to get close to or the entire price that they are looking for, just under seller financed terms.
so for example, I might offer 60,000 cash, and then 100,000 seller financed at 0%.
Once you establish that they will consider seller financing, it's a matter of finding the deal that works in terms of the amount of cash they need and my terms. In the above example, perhaps the seller would counter with 100,000 at 3%. As long as you can make it work with the noi, you just keep adjusting the terms.
I skipped all of the relationship building items because other people have mentioned these above. Just to reiterate though, building that relationship is the most important part...then you make the offer.