Check the cord for breaks or bad connection first, try a new cord. If that fails, check the terminals where the cord plugs into for dirty (corroded) or loose. If that fails, try putting it on high speed and then rapping or smacking on the front and back where the motor and gearbox is and seeing if it starts turning or attempts to turn. This might get the brushes unstuck if they are stuck to the armature.
Assuming the seller is truthful that it is NOS, it likely needs new grease. Grease can get hard or sticky and become superglue and just jam everything in place. I never buy NOS anything because the item still rots and decays even when not used. I have somehow ended up with a lot of Mixmasters and all of them need new carbon brushes and a motor service, they have that dying motor smell to them.
There's a guy on YouTube that is a Mixmaster pro and repairs these all the time
