While I have been beaten out on deals while I am in the car, I have improved my batting average quite a bit by making some reasonable requests of the seller. First, I tell them I will be driving a distance (I usually give them a time, like a two hour drive). I then agree with them on price, usually full price, which I think encourages them to hold the bike. I then share some stories of getting scooped while I have been driving to buy something. I thank them for their agreement, and then I immediately get in the car and drive, right then.
As a seller, I do not negotiate price via email or the phone. I explain to the potential buyer that I want them to see the bike first, make sure it is what they want, it fits right, etc, and then they should feel free to make me an offer in person of what the bike is worth to them. (I leave off my main reason is that most potential buyers never show up, so I consider it a waste of time, and the only one really agreeing to the offer is me the seller. If they show up, I am obligated to honor the deal. If they do not show up, I have no recourse).