The first thing that occurs to me is whether or not there is a shortage of spaces for bona fide visitors.  If not, the next point is why would you give away a valuable resource to a resident just becasue they have more cars than they have spaces for. 

In other words, you could (theoretically) be renting these spaces at the going rate for car spaces in your area and provided you aren't renting the spaces to non-residents, you're benefitting the whole building. 

However, the on major consideration is whther or not your allocation of visitor car spaces is part of the Development Approval for the building.  If it is, you could run foul of your local council because you have breached the terms of the DA. 

How would they ever know?  The first time someone can't get a visitor space or finds that all the allocated spaces have been rented out, they could be on the phone to the council.

But if you can avoid these various pitfalls, my view would be to install pop-up bollards and rent out the spaces with the proviso that they have to be returned for the use of bona fide visitors at times of high usage like Christmas and New Year, for instance.

You'll need  special resolution by-laws to do this properly but I'm sure there are lots of buildings with “informal” arrangements along these lines.