I think the key to any negotiation, is to find common ground and establish some type of trust early on. This can be done by making small talk and asking about any projects the seller is working on, and mention what you are into. If you have kids use them to your advantage, as long as they are respectful and can behave. Most people have a soft spot for respectful kids. Above all be respectful of their time, only do this if you are ready to purchase.