These machines are being sold with older operating systems on them.  They are late 2013 models, and as such, should be capable of running the latest OS, OS 12 Monterey, Apple's newest.  I find it highly suspicious that the sellers haven't upgraded the operating system to OS 12.  Why wouldn't you, if you could, as it would make the machine far more attractive to buyers.  I strongly suspect that if one were to submit the serial number for any of these to an Apple support agent, that agent would not be able to find it in his/her data base.  I bought one & had that exact experience!  When, during the course of supporting a customer's purchase of a new Apple computer, it comes to light the the machine is defective and cannot be repaired to original specs, Apple 'flags' it and removes its serial number from all of their data bases.  They won't give phone support for such a machine, nor will they accept one for repair at an Apple store.  If you decide to try one of these, the first thing you should do upon booting it up, is to ask it for a software update.  If it offers up OS 12, no harm, no foul.  But, if it offers no upgrade to the OS, beyond a version of OS 10, I strongly suggest that you arrange to return it immediately.  If you've already put any personal data on it, boot it in recovery (boot holding down the command and the R keys).  Once it's up, launch disk utility and erase the hard drive.  Then, quit Disk Utility, and from the resulting menu, select reinstall OS.  Let it run its course, but when it finishes, don't do anything to set it up.  Just shut it down and return it.  The seller will receive it void of your files, and in exactly the same condition in which it was delivered.