a display item is 'used' from the point of view of current consumer legislation, but that doesn't affect your rights under the sale of goods act. That legislation covers second-hand items, even if you buy them from a mate in the pub.
Therefore the retailer's liability is unaffected.
The manufacturer's warranty may not be assigned without prior consent however, which means that it is null and void (as your manufacturer said) if goods are sold as secondhand. Had your retailer sought to clarify the position before selling the display monitor the manufacturer might have agreed to assignment.
The fact that you obtained satisfaction makes all this an academic point, but now you know the legal position - the manufacturer was within its rights in stating 'null and void'