I think we are in danger of classifying too many classes as 'lost'. The definition we've used is
So yes to the like of Spearhead, ToY, Unit and no to the Laser II etc as a lost class........those often strange and weird classes of dinghy, that were drawn up by excited designers full of optimism but never became established with the fickle sailing community.
Sometimes it was because they just weren't very good, sometimes because they were way ahead of their times or a bit weird or scary.
However many of them are very interesting boats and important parts and stages of the evolution of dinghy design
As far as eBay and it's impact, personally I think it's a lot wider than just eBay. There are free places to advertise ( http://www.boatsandoutboards.co.uk , http://www.apolloduck.co.uk etc) and class associations have their own websites there's more of a marketplace for advertisers, as opposed to eBay where it costs to sell.
You only have to look at the decline of classified ads in Yachts and Yachting and it's easy to see the attraction of the free advertising in DSM and various websites.