Intesting idea only thing is it will be to hard to manage u would have to rent the sign then keep it up to date remever it when ur done and people will be looking there and not the shops so if u forget to then no one will know ur selling were as it is now they have to look at the shops to see so it does not mater if u forget or not as they will be looking anyways. Also if i made it open so anyone can edit so there is no renting people could remove your post or change it as there is no easy way for me as a GM to tell whats old and outdated signs and whats not.
Ill keep this in mind tho and see if i can come up with an idea but as it is now it feels like its going to be more work then needed. (sorry for the run on lines here im on my phone atm)