According to The Guardian[1]:

In John Markoff 's 2005 book What the Dormouse Said: How the Sixties Counterculture Shaped the Personal Computer Industry (even the book's title is from a hoary old Jefferson Airplane track) he reveals that the world's first online transaction was a drug deal:

In 1971 or 1972, Stanford students using Arpanet accounts at Stanford University's Artificial Intelligence Laboratory engaged in a commercial transaction with their counterparts at Massachussetts Institute of Technology. Before Amazon, before eBay, the seminal act of e-commerce was a drug deal. The students used the network to quietly arrange the sale of an undetermined amount of marijuana.

[1] http://www.theguardian.com/science/2013/apr/19/online-high-n...

Is a footnote needed for everything? From the author's perspective, I think it was assumed that it is well enough known to not need a citation.

And if someone didn't know it, googling "first ecommerce transaction" brings up multiple hits on the first page confirming it.