The Common Property is owned by a Corporation and common law property rights apply irrespective of whether there’s a By-Law.
Neither the Owner who’s selling, nor their selling agent, nor the sign-erecting contractor have the right to erect a sign without the permission of the OC. That would be trespass.
Having said that, a By-Law would spell things out clearly to Owners.
I see nothing wrong with a By-Law covering:
1) Types of “opportunity” (Auction, Lease, Rent or Sale) which are permitted.
2) Where the sign may be erected.
3) How long the sign may remain, in total, but also after contracts signed.
4) Maximum size of sign.
5) Manner of attachment.
6) Prescribed fee (to be paid in advance) for use of Common Property.
With number (6), you’ll soon find out whether selling Owners think such signs are worthwhile, or whether they are just enriching themselves at OC’s expense.
PC, selling Owners will only benefit once each, absentee landlords will benefit many times.