I recently purchased a highrise condo unit during the preconstruction phase. The building is scheduled to be complete in December 2007. It is the first building in what will be a complex consisting of 5 residential high rise buildings. The building i invested in (the first of 5) sold out in a very fast, 7 hours, so interest is high.
I am seriously considering flipping the property but i'm not sure when and how i should market my unit. I'm not sure how to legally assign a property before closing.

You need to read your contract... most newer contracts now have a significant cost to assign prior to initial close.... you could loose the majority of your "profits" back to the developer.

This is something you should have known Prior to making the purchase offer. You may only have a reservation and not an instrument you can assign.