If only. I think it is the show pictures that add the value, with the as featured. There could be a moratorium placed on selling a vehicle with show pictures supporting a sale as that infers some kind of club association. I was surprised to see a guy advertising his 280 Capri on eBay during the NEC show with stand pictures and a come along to the stand to view write up. So, there is no doubt people can and do use shows to shop front their cars and deals can be struck rules or no rules.
Its just as well as a owner of mine for 18 years going on soon to 19 years that thought to sell never crossed my mind, at all and its unlikely it ever will. All offers received, being lots, were politely and firmly refused with a firm not for sale. But the OP could attend any car show as a way of finding out what the marketplace has to offer with a good cross section of owners and cars, from totally honest and stock to completely modified with bolt on anything you could think of and some may then, or very soon after, become available to buy.
Like nicky I'm on the look out for a new Supra. I have negotiated on a stock jspec TT6 with low miles (km's actually) and its offered at a reasonable price. It's not in the colour I wanted so I low balled and had a nibble suggesting a improved offer would secure. If the OP would consider a jspec TT6 he can PM me and I'll send him over the details as it looks a good car from a reputable source. Just not UK original. Only PMs from the OP please as they are considering and may yet come back and accept my low ball offer!