There is more than just having the money on hand.
A few extra tips from an "old dog".
However, if you have not done this before, I will upfront tell you, it is good time to get an experienced collector to be beside you.
Carry your wad of Ben Franklins in your pocket, which you flash "by accident" during conversation IN PERSON when explaining the offset in comparison to how much he can make in comparison to route operation.
DO NOT do this at the location of the machine, but rather at the operator's business location by travelling to meet the owner (not technicians) directly. If you are talking to technician when he is cleaning the game, you are already "doing it all wrong". Don't infer any significant experience "in pinball" to the operator owner, or it will simply be perceived as arrogance.
Always have some "flex" in the amount of value, and never tip your hand on how much you are willing to pay initially.
Learn how to negotiate, know who are really supposed to talk to which is the owner not the minions, understand the game's specific "Achilles heels", request a full technical inspection of the game or get assistance before final negotiation of the price (which is done on site), and know the base value of the machine in above average, fully working condition.
If an agreement is reached, be prepared by having a written printed fillable invoice in your other pocket with a pen which you can both sign at the time, as the operator will most likely not be prepared. This is commonly forgotten, and critical step. Use a witness, if possible, as some operator's can and will attempt to "change the deal" afterwards. Protect your interests, and IMMEDIATELY pick up the game under their supervision meaning have your tools and transport ready. Do not leave and "come back later" as you are looking for trouble for a whole host of reasons beyond this posting.
Addendum: Another collector just located a NIB DE JP that was found in the care of widow of an old operator in the last couple of weeks. He bought the game for $500, although she offered the game to him for free as a friend. Truth is stranger than fiction.