Popular subject of photos like business men and women are selling very well even if they are not collages but single image, but such subjects as backgrounds, textures, grunge and other with more abstract meaning images are selling less.
Collages might be a way to show some particular images in one, it could be a good advertisement because it simple better gets to potential costumer why? They are different, colorful and have many subjects they could be more interesting then regular single image. One texture might be boring and might seem to be needless but four of them in one composition might show that from those images you can create anything you want. Sometime it is not so obvious, so collages compound from our work might promote them.
You can put ID numbers of each image to keywords of collage that potential buyer who is interested in only one could simple find it. Have a fun as I have in creating colleges and promote your self.