I have never bought a pinball machine online, but have dealt with providing evidence that something really is in your possession. The easiest way I can think of to verify that a user has something, is to demand a picture of something random. I would think, "Send me a picture of the machine with a large piece of Broccoli under the glass but on the playfield" would verify that A. The Machine was in their possession B. They have the keys. C. The condition is really as it says and not photos of another machine.
I have seen this method used successfully in "aquarium grow out contests", where you have to prove that a tank is empty at a certain point in time.