When you get a notification that someone wants to buy your work - this means that a user would like to make a purchase but can't because the image is not enabled in your Store. This can be because your Store is not enabled at all, the image is too small to be included for sale, or you have not checked off the 'sell this image' box next to the image in your Store settings.
After you receive this notification, go to 500px.com/store (once store is enabled) and upload the requested image, or mark it for sale.
The photo must be at least 3000px on the longest side. Once the image has been added to your store, the user will receive an automatic notification that this image is now available to purchase. The user can now purchase a canvas print or a digital download of your image. All printing and shipping is taken care of by our professional lab.
Read here for more information on how to sell your images.
Need more help? Email email@example.com or tweet @500px_help