Can eating shark cartilage prevent cancer in humans?

No, at least there is no reliable evidence to suggest that it does, Numerous studies have been conducted to test this and all have shown no significant improvement against a placebo. Shark cartilage can cause some side effects,

Furthermore sharks are under significant threat from fishing with almost half the species considered endangered with extinction or near threatened. We are at the point where species extinction is on the table and unproven psudoscience preying on vulnerable individuals does nothing but increase commercial pressure on an already vulnerable resource.