When deciding to sell your images, you need to decide which licence type you wish to sell them under. You cannot sell your images as royalty free with one library and exclusive with another.
This licence allows the customer to pay a one-off fee and then they can use the image multiple times and in multiple projects within the terms of the licence. The price is determined by the size of the image purchased.
With this licence the customer pays a fee each time they use the image and the customer must specify the intended use, media, territory and duration for each usage. The price is based on the intended use of the image.
In the past if you had an image that contained a recognizable person you had to have a signed model release for every person; otherwise you couldn’t sell the image. Then they introduced editorial licences. An editorial licence allows the library to sell the image with restricted use such as magazines and books.
Not all libraries accept editorial images, but the ones that do need you to provide a detailed caption about the image including the location, date taken and a full description of the image. Search each library’s help section to find examples of their specific requirements. If you don’t get it spot on, they will reject your image.
With this licence the images are exclusive to that particular library. This means that you cannot (and must not) legally place them for sale with any other libraries.