iCloud and AppleID 'married' to Macintosh When purchasing a used Macintosh, what does one need to know (i.e. what will adverse outcome) regarding the Hardware (serial #?) being tie to an ID?

Apple Products: Due to Apple removing the old website they had to check for iCloud or Apple ID accounts tied to devices we cannot be certain whether or not a device was tied to an account in the past. There is a possibility that a device may bring down a lock from a previous owner after we have already processed it. If a device does bring down a lock past our standard 30 day return policy it will not be accepted for return.  

What can be done and what resources are available to mitigate this risk before buying the used hardware?
The tie of an AppleID to a device is associated with the "Find My" capability. On an iPad or iPhone, this tie to the given AppleID makes it so that new users of the device cannot wipe it or change the AppleID without providing the previous user's AppleID login credentials.

The good news is that the tie to an account is not as "strong" on an iMac.  For that device, you can merely reformat the drive and install the desired version of MacOS.