I'm running Server 12 with anywhere from 10-15 stations at a time consisting of a variety of v12, v13, v14 as well as Advanced on the client side.  Recently purchased a few license keys from eBay and I'm getting unique license key errors on my network.  The keys definitely look unique to me, but maybe they're not?  Anyone aware of license key "types" that cannot simultaneously run on the same network even though they are unique?

firstly I am a bit dubious of purchasing license keys of FM on eBay - not sure what the terms of service is for resell or transfer of ownership for licenses keys.

Personally I would not run that many mixed versions of client and server at once - its just too many version differences that will introduce possible corruption in to your solution. Plus there are many security updates that have been made over the years.