Seems like buyer remorse if those are the reasons stated by the buyer. You are not obliged to accept such returns unless your listing policy says you do.
But I would certainly be on the phone to EBay about it. Do you have another witness that saw you open the package to find the shirt? If do ask them to sign a stat dec that says so. If EBay find against you then raise a case with The ACORN. That puts the matter into the hands of the federal police.