Fiverr sales musn’t be conditional upon results, plus there is no way for you to track this unless you have access to sale records and are periodically tracking them. Under your rationale, all a seller would have to do is say “I didn’t get this result.” They could withhold results from you and under no obligation to prove results. And then you would refund them? With this kind of policy, you’re setting yourself up to be scammed, plus giving yourself extra administrative work.
Also, what you are proposing requires a contract within Fiverr that would say what the results are conditional upon, but there is no mechanism in Fiverr to put this into place. Why? Because the ToS and the system itself mean that the contract is a deliverable, NOT a result. To put your policy into place, you’d have to break the ToS.
This kind of policy is fine if you’re an agency, but it does not belong in a marketplace.