Pens matter. Ink does not.

In the age of Amazon, evaluating antitrust concerns is sometimes twisting government officials into knots. That was made clear this week, when the Federal Trade Commission blocked the merger of Office Depot and Staples, in part by drawing elaborate distinctions between who buys pens and printer ink cartridges, and how.

The products are widely available online, of course, including from Amazon, which by some estimates accounts for half of all online shopping and has reshaped the retail industry. But big companies, the F.T.C. argued, like to shop in bulk, and often don’t buy their ink from the same places as people shopping for themselves. For bulk purchases of Post-it Notes and pens, Office Depot and Staples are the primary options.

The trade commission’s argument underscores just how complicated it has become to judge Amazon’s place in the retail industry. To put it simply: The agency does not think that the company benefits all customers equally.

The decision also shows how difficult it is for regulators to keep up with industries being reshaped almost daily by technology.