Advertising on a facebook group?

[Jake]

I am the sole admin for a large facebook group for my college class (around 3100 people) and think that nearby student apartments would be interested in advertising on that page (say, if I made the banner photo their housing complex and address or something). The facebook group was not created by the school, it was created by me and grew by word of mouth and social networking. I'm just wondering if anyone knows if this is okay, as in legal to do on facebook, selling advertising space on the group on facebook group I have created?

If it's not okay, why?
If it is, advice? Is it a good idea?
thank you