You really can't get rid of scratches in glass, unless you have a diamond coated polisher (as in making lenses, etc). There are some DIY kits on the market - but I rather doubt that you get good result with just the grit & elbow grease.
You should go & see where the scratches are & if you can live with them. <PS if the scratches are only at the front - turn the tank around, then they will be at the back!> Also $50 is not a good deal if the tank is really scratched up. I got a brand new 80 gal
for $100 on sale ... so if you are patient & wait around, you can do better.