Stamp catalogue prices are not considered reliable as they are nothing more than estimates at the top end and represent a retail selling price at the bottom end of the market. Auction realisations may be more reliable but are difficult to use as the investor has to personally analyse the realisations from many auctions over a long period of time in order to come to any useful conclusions. While most trading in shares is on a recognised stock exchange and takes place transparently in public, that is not the case with stamps where only auction transactions take place in public view.