There is no rule for money back guarantees, and some sellers offer this while others don’t, although I don’t see how money back guarantees would be in line with Fiverr’s TOS, because it doesn’t feel right to circumvent Fiverr’s ways. Offering money back guarantee is like trying to prevent negative reviews, which is not allowed as far as I know.
@soload Politely ask your buyer to provide a valid reason for why they are asking a refund, because without a good reason asking for a refund is against the rules. As long as your delivery matches the description of your gig, you are entitled to keep the revenues associated with that order, for the buyer has paid a service, your time and expertise, which they have acquired, and without a valid reason it’s like trying to get free work.
If the buyer still refuses to give a valid reason, you should ask Customer Support for further help.