Unless the original welder, frame builder, design engineer, weighed in, or better pictures with comparisons to historical pictures, there seems to be no way to tell.
The seller does not offer any evidence, just an approximate age and the claim:
Originally Posted by CL New Haven, CT
Vintage prototype Cannondale 12 speed all aluminum road bike with Campagnolo components. 66cm height-great for taller rider 6'3" - 6'5". Although this bike is roughly 35 yrs old, it has kept inside and never been ridden other than test rides. Completely original.
When a claim such as that one is made, it should be supported with at least an interesting back-story,

even if the
Back-
Story is just a bunch of BS, at least make the ad more enjoyable!