Without seeing the tank, if I were to repair it, it would drill a small hole at either end of the crack, fefore I patched it, to discourage it from spreading.
If I was thinking of buying the tank, I would ask myself why the seller didn’t repair it. I would assume a tank without a crack is worth more than a tank with one.
Finally, what are you planning on using tank for? Humans, animals, garden, or something else?
I am finding the price for a new black water tank that looks similar to be in the $2000.00 range.