All photographers at Mostphotos has contractually agreed that the images they upload as commercial royalty free always have the model's written approval. This means that you can buy the images in good faith. In case of a legal dispute it is always the photographer that will be deemed responsible.
If you want you can always contact the photographer and ask to see the model release before you purchase the image, as the releases are not in need to be uploaded to Mostphotos, only to be available if requested.
Contact firstname.lastname@example.org if you have any more questions or need any help.