On 500px you can add a preview with your own watermark. Using a batch processor like Photoshop CS6 will add the watermark automatically. It just requires a moment to set up, but then it will run in the background when you are making a coffee. Once you have your batch of previews, you can upload them to 500px, and attached the full res images. Buyers will see the watermarked preview, but they will purchase the full res watermark free version.