I have 40+ pages of pets in my rancher shop, and sometimes it's hard to keep track of which pets I've priced and the newer addictions. Having an option to display them by price would make it easy to see which have no price set at '0' and input them if we so choose. This would avoid having to sort through each page to see if there were any unpriced pets that I may have missed.
This could be applied to merchant shops as well.