System builder volume licence

I just got hired to be pc tech for a internet cafe bit rusty on computer as I had gone in to another field of work for a while...

anyway the question is their previous tech had build and sold them computers claimed to have a systems builders license and from what I understand each computer should have an oem sticker which doesn't as I come to find out later that the Key used is a Volume license key that is no longer valid /Illegal key

so the owners call up the previous tech and he then gives them a copy of xp witch has a volume license key Legal to him I am guessing
now my question is If I install this where it is registered to the previous guy and being volume license is this even legal ? and being volume license could die at any point ?
and shouldn't this guy be made to buy OEM copies to install on each computer or pay for a volume/corp license in this companies name ?