Someone put a bottle of acidic toilet bowl cleaner on my marble floor and countertop. The marble is now etched. It's going to cost something like $700 to sand it and polish it. These are not big areas. Have any of you attempted to repair something like this yourself? I would imagine I might be able to sand it down with progressively finer grit sandpaper? Bad idea?

How big is the spot? Assuming this is polished marble (not honed, if it's smooth and shiny it's polished), you can use a marble polishing powder like MB Zero+ or Fila Polished Marble Restorer

Clean and dry the area, lightly cover the entire area, mist water over the powder, rub vigorously in circular patterns for 45-60 seconds. If the spot is still visible, let it dry (so you can better see the spots) and do it again.

If the spot is more than about 6"x6" you may need to sand, but smaller than that you can just polish it out