Maierhofer, 27, joined the club from Rapid Vienna for £1.8 million last summer but suffered form and fitness problems in England and was sent out on loan to Bristol City.
"Stefan is for sale. He is, yes," McCarthy said. "I tell the players if they can go and then I guess they tell their agents who go and drum up a bit of business - and get the business done.
"There is a bit of interest in Stefan from abroad."