As everyone else has said.
Smile, be polite. Make small talk. Hopefully it is not a deadly serious business interaction that will make or break either party.
Don't spend your entire time finding problems with the machine and pointing them out. Ask about when they got it, talk about how you enjoy playing that pin, talk about the old car or other random thing in the garage/basement/family room the pin is in. Positive, smiley, good feelings.
I am blown away when people tell stories of potential buyers coming and bashing the machines condition the instant they show up and generally being jerks. Sometimes the buyer will have driven hours to come and see the pins and will leave without the pin for a $100-200 disagreement in price that the seller would have dropped to if the buyer was not such a jerk.
A good offer:
"I am sorry I can't offer you your full asking price but this is what I can pay for your _____."
Maybe you "can" only pay that much because you are out of money or because you don't want to overpay. The reason does not matter and you don't have to say why.
Have the cash, the cart and truck with you to take the pin right then. "I will pay you cash and get this out of here right now."
Feel the seller out on the phone if you have to drive very far. If you are split by to much in price then be polite, let them know that you hope they get the price they are asking, leave your name and number and say you have the cash ready and can pick the game up promptly if can things change in the future.