There are several online platforms where photographers can sell their photographs, including:
Shutterstock: A stock photography website where photographers can upload and sell their photos.
iStock: Another stock photography website, owned by Shutterstock, where photographers can upload and sell their photos.
Adobe Stock: A stock photography website owned by Adobe, where photographers can upload and sell their photos.
Alamy: A stock photography and video website where photographers can upload and sell their photos.
500px: A photography community where photographers can upload and sell their photos.
Etsy: An e-commerce platform where photographers can create their own store to sell their photos
Smug Mug : A photo-sharing website and online community where photographers can create their own website to sell their photos.
Photo shelter: A website that enables photographers to create their own website and sell their photos
These are some of the popular online platforms for selling photographs, there are many other platforms depending on the niche you are targeting.