"He causes all, both small and great, rich and poor, free and slave, to receive a mark in their right hand or in their foreheads, and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name. Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666" (Revelation 13:16-18).
Referring to the last days, this could only be speaking of a cashless society which we have yet to see, but are heading towards. Otherwise, we could still buy or sell without the mark, amongst others, if physical money was still currency. RFID microchip implant technology will be the future of a one world cashless society, containing digital currency. Revelation 13:11-18 tells us that a false prophet will arise on the world scene doing miracles before men, deceiving them to receive this Mark. Trust in Jesus. AT ALL COSTS, DO NOT TAKE IT!
"Then a third angel followed them, saying with a loud voice, “If anyone worships the beast and his image, and receives his mark on his forehead or on his hand, he himself shall also drink of the wine of the wrath of God, which is poured out full strength into the cup of His indignation. He shall be tormented with fire and brimstone in the presence of the holy angels and in the presence of the Lamb. And the smoke of their torment ascends forever and ever; and they have no rest day or night, who worship the beast and his image, and whoever receives the mark of his name” (Rev. 14:9-11).