Not everyone, either buying or selling, is as clueful as thee and me.
Indeed, but in a case like this I'd suggest a discussion of how you can go to Settings, System, About Pane, then look under the Windows Specifications section to determine exactly what just installed on the system. If it's precisely what you had intended to install, then it's done since you witnessed the whole thing, regardless of who had the install media.
I am simply not willing to believe that most folks selling their own computer when they've gotten a new one are skilled or nefarious enough to put in a compromised Windows 10. The wrong edition, sure, but that's easy enough to check before throwing the baby out with the bath water. Were it a case that they installed Win10 Home in the correct Version in my presence and I wanted Pro, I wouldn't wipe and reinstall. I'd use Settings, Update & Security, Activation Pane,
Change Product Key link to update to the Pro Edition.
Windows 10 gives lots of very easy ways to verify things that took a lot more effort in the past. I prefer to teach people to use them, when needed, before taking
the thermonuclear option of a completely clean reinstall. I've also sold machines to people with things like MS Office installed in a release that still worked perfectly well, but that I was not intending to update or re-use on another machine. Many would prefer to keep that kind of software, and I don't blame them.