DO your research first...I looked at a unit from a dealer they were asking $49,000. I said the absolute most it was worth was $25,000 any they came back with, we can do that. I didn't buy it. I prefer private sellers you get a good sense about them and how they treated things when you spend an hour or two with them asking questions an looking over the unit and they will have the paperwork to back things up, at dealer (unless all work was done there) you get nothing.
Basically if your not uncomfortable with how low your first offer is, you didn't start low enough.
