The Owners Corporation has the right to allow the placement of signs as it sees fit. They own the common property, not the real estate agent selling the property. As an EC member I have always believed in allowing owners to facilitate the sale of their property by allowing them or their agent to erect a sign when requested. But we determine where the sign will go. Of course a lot of agents will just plonk a sign on the common property without the common courtesy of asking.
When the property is sold many agents would leave the sign there forever, with a sold sticker on it. It then serves only to advertise the agency, not the property for sale. In such cases I have had no hesitation in pulling the sign out of the ground and leaving it for the agent to collect. In one case I did this and the sign was promptly replaced by another with a sold sticker over it! In that case I rang the agent concerned and asked them to collect the two signs.
As far as legal ramifications are concerned, my interpretation would be that as long as you don’t destroy the sign or permanently remove it from the agent’s possession you couldn’t be charged with theft. In any case the agent wouldn’t want to be out of favour with the residents as a whole by instigating legal action. There’s no way I would bother to deliver a sign to the agent’s premises, they put it in, they can collect.