An HOA can limit any signage that they choose. Many will prohibit signage that is in common areas, but not those that are in your private space. If you have a balcony, you may consider placing a sign there, but first check with the HOA to see if the sign can be attached to a railing or if you have to have it free-standing (you can put a stake sign in a planter pot). Window signs are also generally allowed, but again, check your HOAs rules.
Statistics show that For Sale signs account for very few home sales, but they are helpful in assisting people to find your home once they are in your development. They also help your neighbors who may be looking to move into a new unit in your development - they may be renters, or they may be wanting to move up to a larger unit.
That said, the internet is your best advertising resource if used properly. Most Realtors only use a fraction of the internet's capabilities, so be sure to hire a Realtor who knows how to maximize your home's exposure online.