On a similar theme to my article on metaphorical postcards (This is the metaphorical prints article ) - it is important that when showing a photograph you do not show a rank and file of postcards, stamps, etc or even worse a jumbled heap but only the best one. The reasoning behind this is because the ident is very small and a lot of the detail will be missed out on. Another reason is when you say you have 16 postcards for sale, most people will know what a group of 16 postcards looks like from a distance, but people won't want to look at them from a distance. Of course the disadvantage is the buyer sees one of the items instead of all of them, but as its the best one they should know what to expect.
But how do you chose the best one? Well say you have postcards of a number of cities-the best one would be the one the most people want, the most populated city in other words. If it is a group of books in a series usually the first in the series represents the group. The only exception is when you include a really up to date book, in which case you should go with that. When selling an artist's work you should try to look for the most well known example of their work-for instance if you were selling Donald McGill prints you obviously want a risque picture.
The whole point of this article is to try and get your ident to look as professional as possible. Hopefully then you'll get more sellers.