I think several people here have asked, but I will too: you ARE actively looking for another job, right?
This sounds like what happens at our neighborhood association. It's a bunch of little red hens - willing to go to the party, but not willing to help plan them or staff them.
In the context of work, though, you obviously should have more support from your coworkers and the supervisors who get to decide who they can and cannot spare for an hour at the end of the day.
It's hard for me to imagine that these events bring a lot of value if your coworkers aren't using them to interact with customers or whatever it is they're supposed to be doing. But nobody has any incentive to help you, since you're in a position where you have to get it all done yourself, all the time.
If there was a way to make this mandatory (everyone has to work one event once a year) or provide some sort of cookie to people (an extra vacation day for people who put in eight hours of help), then you might get more volunteers. People in sales especially need to see a tangible benefit to them for anything that doesn't lead directly to a commission.
Also, is there any way to scale these things down, so that it's manageable for one person?
This is a stupid idea, but do you have any friends or family who would be willing to help you? Maybe if people actually saw a stranger volunteering just because they love you, they'd be inclined to believe that you really do need the help after all.
Of course, you can always let an event fail - sounds like it's bound to happen someday anyway.