Dell Service tags not recognized on multiple E514dw Printers

Hi all - I trade in computer hardware and spares and have recently purchased a large batch of Dell E514dw that were supposed to be new retail sealed units, however on delivery to us the boxes are clearly opened (or not sealed) but more concerning is that I have tried entering 4 of the tag numbers into Dell and they don’t even recognize them. Any ideas? I am wondering were these units supposed to be for resale or are they refurbished and so no warranty is honored thus the tag number snot being valid? - Many thanks in advance.