The unwritten rules of Craigslist are no refunds, no returns, and no contract. Most buyers & sellers know this.
So your buyer has nor recourse if you decide not to refund money. Of course, if you want to do so, that is because you are generous, or sincere, and want to do the right thing - not because you are obligated or legally required.
Craigslist will not be involved in such an issue, you will have to settle it between both of you.