mmmm, looks like that whole corner is not being supported at all, and all that is holding the water in is the silicone in that corner. Could be pretty dodgy, but yet again, could last forever...
Cutting the old silicone away from the corner, and siliconing a patch on the inside may work, or cutting away the silicone on the outside and putting a small piece in that corner... wont do alot but may give a bit more support for the side/front.
Either way, Id talk to the seller, and see what they say.
EDIT: the best option would be to cut the old bottom off and siliconing a new piece of glass on the bottom. at least it will give peace of mind, and will be alot cheaper than loosing a whole tank load of fish, plants, and burning out heaters or filters.