Commissioning questions

[akktri.livejournal.com]

I've got a commission idea, but I don't want to get swindled.
I would like some advice about this before I do anything.
Here's the idea:
I go on a furry auction and have people buy advertising space in my furry comic book. They'd be sponsors, basically. They would be limited to four pages, and charged extra for color. I'd print about 10 comics with their ad in it.

Unfortunately, I sense that there are way too many ways for people to take advantage of me when I'm doing this auction. How do I go about making and enforcing a contract for this situation?
I suppose making long distance phone calls to the buyer is also a requirement...