Not deadbeats, "change their mind" types.
Common as cotton woven t-shirts.
Nostalgia can be a powerful foe.

Be ready to buy and pick up immediately, or not at all, particularly from seller to whom you have not dealt with before.
It sounds like things were planned in months, not days.
Learn not to try and lowball, if a game is in the condition as expected, as this can sometimes be offending.
Long time collectors know their prices, and don't shank others. New high end rollers can, especially with "mods", powder coating, or chrome, or simply try and flip games for more money a few months later.

The strongest advice I can give is do not give a person a chance to acquire the disease of getting seller remorse, as it can only take a few hours, not days.

My only exceptions above are between networked pinball acquaintances, museums, and friends, for "holdovers".
Trust will build more trust.
Reversely, buyer flake out will not lead to better future deals from the same parties.