I think the reason that you don't see a lot of Sunbeams on a site like eBay is that eBay tends to be more of a generic "used car lot". The cars that end up there are either extremely unusual (I sold my race prepped Imp on eBay), or just average examples. In each case, the seller is looking for the maximum exposure to the selling public. I suspect that the nicely restored "original" cars more often than not are sold within the Sunbeam community/clubs where those with greater interest in the car are already gathered.
Obviously, as there were fewer Sunbeams built than say MGs, Triumphs or even Jags, there will be proportionally fewer to be seen for sale and at car events. However, the truly exquisite Sunbeams are now in the hands of the enthusiasts, most of whom belong to the various Sunbeam organizations. Just attend a Sunbeam specific event (Uniteds on both coasts, or SUNI) and it is there that you will see plenty of original cars. True, it is said that Sunbeams have been modified more than some other marques, but I believe that proportionally there are still a lot of "original" examples out there.