Fine Art America is the world's most powerful sales and marketing tool for photographers and visual artists.
Simply open an account, upload your images, set your prices for all our available products, and you're instantly in business! FAA provides you with an e-commerce website, fulfills your orders for you, and sends you your profits each month.
An App, in the case of Etsy, would allow you to browse the store without having to actually use your phone browser. It's a more simplified, compact way of surfing the store. There are literally tens of thousands of Apps for things like banking, shopping, restaurant and movie reviews, etc.
Of course, you'll need a data plan through your carrier in order to do these things.