If such a situation occurs, the buyer can contact Customer Support and report the dishonest seller. Fiverr can easily facilitate a cancellation and refund the buyer's money, as well as take action by banning the dishonest seller.
What if they say they don't like my work and it doesn't meet their expectations? I won't force the buyer if they genuinely don't like it. However, they could potentially lie about not liking it and request a cancelation while still keeping my source file on their computer.