First-sale doctrine
The rule that a copyright holder loses control of a copy once it has been sold — and the rule the software industry has spent decades routing around.
It is why you can sell a used book without the publisher's permission. In the US it dates back to Bobbs-Merrill v. Straus in 1908; EU law calls it exhaustion of the distribution right. Software's counter-move is simple: you aren't sold a copy, you're granted a licence. That is why every EULA chooses its verbs so carefully.
Courts haven't bought it entirely. In UsedSoft v. Oracle (2012) the EU Court of Justice held that a downloaded licence of unlimited duration is exhausted just like a disc, so Oracle couldn't block resale. In the US, Vernor v. Autodesk (2010) went the other way. E-books fell outside the rule in the Tom Kabinet case of 2019, and account-bound games settle the matter technically: there is nothing left to resell.