I have experienced 1 time,
the buyer thought my model was real thing so they buy it, mostly from women. But i have to return the money because the buyer haven't still downloaded it. This happen in other 3dmarket site because this site keep money and take responbility about download models.
On your case, if the buyer haven't downloaded your model, i think you should return the money to him.
If he have downloaded it, so he have to pay for that because he didn't read careful about description of that model, but it depend on your generous, or you can deal with him to repay 50% of the money because of his mistaken.