Quick Window checkout module.

I have been searching and I haven't been able to find anything that would work with what I need. I have two sites, one is a open cart store and the other is an auction site. What I need it to do, is from the auction side be able to open a checkout window for an item on the shopping cart side without leaving the auction site. For example, if I want to auction off an item, but need to buy the item first, I can click right there on the item from the auction site and checkout in a little window that is actually being sold from the open cart site without leaving the auction site. I want it to be as seamless as possible. Does anyone know if any type of module or extension exits that will be able to do this? If not, anyone have any ideas how this can be done?