...is not demanded until a sale is made

fotolibra does not demand exclusivity on royalty-free images uploaded to the site

However when a rights-managed image is sold, it is the copyright owner / contributor's responsibility to remove that image from sale through other sources in accordance with the terms of the sale.

For example, if fotolibra sell the world jigsaw rights to an image, that image may not be sold by another picture library for jigsaw use during the period of the agreement.