Why didn't Sanderson sign them?
Why?
Because Sanderson couldn't do it.
"Owen, who knew your brother in Arizona, signed them, because he knew how to imitate your brother's writing.

Get that! Owen signed a bank receipt for the money old Bransford had in the bank.

Owen got it and gave it to me.

He was so drunk he didn't know what he was doing, but he could imitate your brother's writing, all right." "You've got the money ?" gasped the girl.
Again Dale laughed, mockingly.

"Yep," he said, "I've got it.