Do you mean Plano lenses glasses, or non prescription lenses glasses? Technically, the plano glasses won't cause damage to your eyes since the lenses have no prescription at all. But you still need to go a professional shop to buy the glasses so as to make sure the glasses lenses are zero power. That because some cheaply glasses lenses might have distortions that may hurt your eyes for long time wearing.
No. Plain lens glasses are also called non prescription glasses. I think they won't cause damage to your eyes. However, when you get non prescription glasses, make sure they fit you properly. Even if you don't need prescription glasses, you have to have your eyes checked so that you can get glasses with right size. non prescription glasses that don't fit you properly may make you feel uncomfortable.