That is (as The Public Image shared) a quote from Revelations 13:17 ("so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name.") I am not a Bible scholar, yet familiar with history. In Rome the poor did not have money to buy and did not have products to sell. The beast and the mark some say alludes to government, taxation, and privilege. Again, not a Biblical scholar myself.
I draw conclusions with a parallel being another verse Proverbs 22:7 "The rich ruleth over the poor, and the borrower is servant to the lender." Both seem to offer references toward those of riches and wealth with property and riches. Both are protected by government. Privilege as a buyer or seller is governed.
By no means am I inferring government is the 'beast' or the mark is its product, since not being a bible scholar. Even as suggested with the question becoming an eBay buyer and seller it is governed by membership, operating terms (even more so since purchasing is based on PayPal and selling too), and of course the actions of both selling and buying. The 'mark' could be (again a parallel only of thought) is membership of the governing elements.
Bartering is the exchange of goods. Money is only a means of exchanging like values with currency. Currency is just that it is a current in action the same as electricity some suggest. It is fluid and dynamic with values for worth. Electronic transfers is exchanging currency whether between nations or within intrastate or interstate markets.
Just some food for thought pondering the broad stroke of the brush. The source Revelations has immense usage of language and literature presenting its mysticism with prophecy. I tend to consider it prophetic, again not a bible scholar, with relevancy to the present contrasting history and offering a perspective (projection) for the future. I look for common sense meaning with those historical usages of language - grammar, literature - allegory, alliteration, euphuism, hyperbole, imagery, and etc, combined with the current times of history. That is when I may see some sense occurring and again I am not a bible scholar.
A note of oddity or not is the number 6 is the first perfect number in mathematics rooted with Greek thought around 100AD. Could we say those numbers really meant perfect, perfect, perfect? I dun'no . . .