No one can buy or sell unless he has the mark: here buy or sell includes business of any kind that involves exchange of money. In some languages it may be more natural to say “Only those who had this mark could buy or sell.”

The name of the beast: that is, the name of the first beast.

The number of its name: in some languages, including Hebrew and Greek, numbers were represented by letters, and each letter had a numerical value. The number of a name would be the sum total of the numerical value of the letters of that name.

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .