How to persuade top-designers?

Hi guys,

I'm soon launching a startup business that will be selling posters online. We want to create a one-stop place for people who want only the best and most colorful posters. To do this we will be inviting top-designers from all over the world to create the designs for us.

We are having some trouble persuading designers from communities such as Dribbble.com - and we believe the main reason is because we don't understand designers, what really motivates them.

So please share your experience with getting designers on board, or maybe even better; what you as a designer are looking for in a collaboration!

Thanks a bunch.