Rev 13:17; and he provides that no one will be able to buy or to sell, except the one who has the mark, either the name of the beast or the number of his name.
Rev 14:9; Then another angel, a third one, followed them, saying with a loud voice, "If anyone worships the beast and his image, and receives a mark on his forehead or on his hand,
During the great tribulation... no one will be able to buy or sell anything, unless they have the mark of the beast. Certainly you can buy without cash, currency.
You can trade things. You can give gold. You can give cash. You can use debit or credit cards. But in the last days you won't able to use any of these things (even cash currency) unless you take the mark.
Also it seems the "mark".. is just that... a mark of some kind. On your hand or on your forehead. Taping a dollar bill to your forehead is not the mark of the beast.