scratch will be fine ^ just dont have too much lighting, so people wont see the scratch. think of it as your car with scratches, it will run, but looks ugly lol
Smart thing fill it with water and see what happens. Any sign of leakage I would dump the tank better now than later with expensive fish or expensive water damage.
Watch out for cracks if it is one. I bought a used 50 gallon before and after three months, it cracked straight across and dumped 50 gallons of water in the floor in about 2 minutes.