If slomacuser is correct and this is a Radius screen spanning external monitor output port as demonstrated in that video, then this indeed may be a rare and therefore suitably priced Plus. As far as I knew, screen spanning was not possible until the SE & II. I knew the early Macs could easily have external video, but that it was usually just for mirroring what was on the built-in monitor.

Thanks for that video. That's amazing technology for 1986. The guy on eBay would do well to try and figure out what he's got rather than re-listing it until his eBay listing fees negate any of the inflated profit he hopes to earn. Then again maybe he's got nothing and knows it and also knows a fool and his money are often quickly parted on eBay.