Postcard Entropy

I sell postcards online. I have to keep the inventory organized - I'm constantly adding new, removing old, changing prices etc. For the most part I sell standard sized USA views, and organize them by state. Here's the thing I've noticed - when a person buys a card, I can guess the general location of the card by the price I charged. I tend to lower prices of cards when they've been for sale for awhile, and if someone buys a card at a lower price, I know its going to be near the back of the group. When you have a couple of hundred Florida cards for sale, that information can save you a lot of time. Now, my question to myself is, why is that? Why do older cards gravitate to the back? Because I put newer cards in the front? Well that's the obvious answer, but things arent always as obvious as they appear, and it completely ignores physics. I think there's a higher law here. I know very little about physics, so I'm not sure how to articulate it. Of course Newton's laws & some of Einstein's theories are stated in remarkably simple language, its just the mathematics to prove it is mind blowing. Leo's law goes something like this: the longer a postcard stays in inventory, the greater the tendency for that postcard to move to the back of its group. Now is there anyway to make this statement more generic, to make it apply to nature as a whole. Does it work with electrons? Is it related to the general entropy that all things experience? Am I observing a force of nature? Does this have any larger relevance to science or economics? Heck, I'll never know. I never liked science or economics.