Windows 7, for example, has the following provision. This means that the seller must be the first user, and provide you with any media/collateral, and a proof that they purchased the software.
You don't have any way of verifying that they are no longer using the software, there is no mechanism to do this.
From the Windows 7 EULA at
TRANSFER TO A THIRD PARTY.
a. Software Other Than Windows Anytime Upgrade. The first user of the software may make
a one time transfer of the software and this agreement, by transferring the original media, the
certificate of authenticity, the product key and the proof of purchase directly to a third party. The
first user must remove the software before transferring it separately from the computer. The first
user may not retain any copies of the software.