This was an issue for a while where I am. A particular owner got particularly upset about for sale signs at the entrance to the development. The EC at the time found that Real Estate Agents have a code of conduct about where they place them and how long they are left after a sale. Obviously they like to see ‘SOLD’ across their signs for as long as possible advertising how well they are doing at selling. It would be worth finding out the details of that code and insisting on compliance with that at the least.
I would not assume all owners are offended by the signs. Any owner who is selling will suddenly find themselves wanting the most prominent advertising possible and with a change of circumstances that could be any of us!