Three of them on eBay right now!
http://www.ebay.co.uk/itm/Sinclair-C5-New-Unused-All-Original-Parts-Never-Been-Assembled-/190695376847?pt=UK_Collectables_Vintage_RL&hash=item2c665487cf
http://www.ebay.co.uk/itm/ORIGINAL-SINCLAIR-C5-NEVER-BEEN-REMOVED-FROM-ITS-FACTORY-BOX-/330754331784?pt=UK_Collectables_Vintage_RL&hash=item4d027e9488
http://www.ebay.co.uk/itm/SINCLAIR-C5-BRAND-NEW-IN-FACTORY-BOX-/120933987226?pt=UK_Collectables_Vintage_RL&hash=item1c283a339a
..this has been happening since I first stumbled into the world of the Sinclair c5 .
Possibly the largest stash of boxed new c5's was discovered by Mark ,previous owner of c5alive, who met a guy that had 20 of them in his warehouse. Mark purchased the lot and sold them all off.
Over time I dont doubt a lot of people have purchased one or more with the intention of stashing them away and selling them for a huge profit later on when they become collectors items. Sadly even 27 years later the anticipated massive increase in value hasn't really happened as there are just so many of them around still.
And if you buy one you still have the age old dilema of what do you do with it ? Use it and devalue it by 75 per cent or store it away again.