i want people to bid for my ads

hello folks. My site’s getting inquiries for advertising space.

I’ve decided to go the completely transparent route and make them bid for it, with a minimum bid (say, $50.00) to start with, and the highest getting the space obviously.

would anyone know of a plugin, php hack, script or any such sort of program I could use to do this? I’d like to post it on my wp site so everyone can see.

Any help much appreciated. Many thanks.