Have you thought about patching it with a piece of glass glued to the inside of the tank. I guess I need to add a disclaimer, I would only try it if the tank was in a basement.
I cant tell from the picture if that's a back ground or the glass is painted. I was thinking you patch it and make that the backside of the tank.
Another alternative would be to buy a new sheet of glass. I don't know if that's cost effective or if you have the time or even want to try such a thing.
I'm thinking that a crack like that was probably from something getting dropped on the rim. Have you tried contacting the seller,anything is worth a try at this point.
If you decide that it's a lost cause for repairing it, remember that people that keep reptiles are always looking for tanks too and they don't care whether it's cracked so you could probably get back what you paid for it.