I generally don't respond to an ad unless I have a good idea of what I want to pay and it meshes with the seller. Sometimes, though, you don't have a lot of choice in it. The very first real pins I bought were a Gorgar and Viking, pulled out of an actual barn. The pics weren't great, and they didn't know how to open the machine to show the boards and interior. So, I tell them that I'll drive up(it's a 1.75 hour drive one way) and bring cash, but I'm not making a final offer until I get a good look. I ended up picking up both games for the price of one.
The other one that comes to mind was an overpriced(like, WAY overpriced) game in a neighboring town. It was floating on ebay for a while, so I eventually asked some questions. Turns out, he was fishing for offers and mentioned he had two more machines. I ended up checking them out and making an offer on all three. Picked them up for less than the original listing. He was happy, I was happy.