Maybe if you plan on selling a machine, there should be a specific "discounted" vendor space bought. These spaces should be grouped together. And the machine should be on free play for attendees. But in this vendor area, the machine are free to come & go as pleased.
The rest of the free play spaces should be for machines that are not for sale & need to stay for the entire show. Or if you want to make them available for sale, they still must stay the entire show.
My 2 cents & suggestions.