It really seems to me that D.B. Cooper was extremely familiar with the airplane, the airliner, the airport, the geography of the area, and the workings of the crew. This is all backed by testimony from various witnesses.(he knew that the 727 could take off with the stairs down, he used terms (when describing parts of the 727) that usually only the crew would use(he called the telephone and stairs by the same terms that the crew would call them by) he pointed to landmarks from the sky, he asked that flaps be set to 15 degrees(not common knowledge), he seemed to have knowledge of how the fuel truck operated(he talked about the pump freezing up or something to that) he knew Tina wouldn't need to worry about being sucked out when opening stairway, he knew they could get the parachutes locally.
All of this (plus other stuff I'm probably forgetting) is the reason I believe he was a frequent (or at least somewhat frequent) flier in and out of this section of the country. He seemed very comfortable with and also familiar with everything going on around him. This is also reasoning for me to believe he was wearing his everyday clothes.( maybe not the raincoat). I believe he knew he could blend in and not really be noticed. He took steps to keep himself separated from the other passengers and crew members. Think about it, if you would wear makeup and that be noticed by someone, wouldn't that risk drawing attention to yourself. I just wanted to clear up the reasoning behind me believing what I do.
That being said, I would love to hear from the thrift store owner.