zillow.com for comps.
I was the lowest of 3 bidders when I bought the house I'm in now. Why did they sell to me? They liked me. They liked my wife, and we brought my adorable daughter, and they liked her. You probably think this is crazy, but it was a corporate move and I don't think they were sweating the money, and the seller's realtor lives next door (which is how I know all this).
Anyway, it can't hurt to just be really nice. It's a house to you, it's a home to them. They probably want somebody to continue taking care of it after they are gone. They might also feel some responsibility to the neighbors.
I sold my house to the second highest bidder. The main reason was that they were older, had more money to put down, and seemed like a less risky choice.
The high bidders were going non-conventional via a government program, which would have most likely become a morass of paperwork and red tape by the time we closed. It would have bumped my closing out an additional month. They were also borrowing the down payment from family. I didn't want the hassle. It was worth a few thousand less to go with a more stable buyer.