of course sin is involved, but you have not proven the mark is not physical. The mark on the 144,000 is physical. the mark on Cain was physical.As I showed in my first 9 posts the mark, the name, the number, and the image are connected to sin and to the sin nature in man.
The buying and selling aspect of sin is the buying and selling of souls. I covered all of this in my first 9 posts, proving all my points by the words of God.