This is the correct answer. The mark cannot be physical. If it were, it could be forcibly applied to a Christian. And since we know all who have the mark will suffer eternally under God's wrath (Revelation 14:9-11), it doesn't make good explanatory sense for it to be something that can be rubber stamped.
As in Deuteronomy, the hands symbolize action; the forehead symbolizes thoughts. Our thoughts and actions mark our external allegiance to either God or Satan. It will be obvious who is a follower of the Beast by their loyal, consistent, and wholehearted commitment to him.
Deuteronomy 6:4-8
You shall love the LORD your God with all your heart and with all your soul and with all your might. And these words that I command you today shall be on your heart. You shall teach them diligently to your children, and shall talk of them when you sit in your house, and when you walk by the way, and when you lie down, and when you rise. You shall bind them as a sign on your hand, and they shall be as frontlets between your eyes.
Deuteronomy 1:18
You shall therefore lay up these words of mine in your heart and in your soul, and you shall bind them as a sign on your hand, and they shall be as frontlets between your eyes.
"What was given originally as a metaphor became for later Jews a literal injunction. The present passage, along with 11:13–21 was written on small scrolls, placed in small leather containers and bound on the forehead and the left arm when the Shema was recited. The origin of the phylacteries (cf. Matt. 23:5) lies in this literalism. The phylacteries were worn by every male Jew during the time of morning prayer except on the sabbath and on festival days, which were signs in themselves. A further practice developed, that of enclosing these four passages in a small container for attaching to the doorpost of one’s house (the mĕzûzâ). Ancient copies of these documents have been found in the Qumran caves and elsewhere. Clearly such a practice might well have had deep significance for some people. The small passages of Scripture were ‘signs’ standing for the whole body of the law which was to be observed and taught. But where the practice descended into one of legalism, it destroyed the whole spirit of the ancient injunction. It was love toward God and the remembrance of all his past mercies that moved men to obedience. These were signs enough, apart from any physical reminders. The recollection of the saving acts of God and the declaration of his covenant demands would suffice to keep faith and allegiance alive."
J. A. Thompson, Deuteronomy: An Introduction and Commentary, vol. 5, Tyndale Old Testament Commentaries (Downers Grove, IL: InterVarsity Press, 1974), 139–140.
As for being prohibited from buying and selling, I presume this mark plays an economic role through the above mentioned allegiance and not via the actual mechanism of monetary transaction.