Ask HN: Please evaluate my web site idea

I've had an idea for a website/startup for a little while which seems fairly obvious to me, and yet I've never seen the idea put into practice. I'm curious as to whether this has already been implemented, or if my thinking is flawed in some way and it's actually a bad idea. I'd love to know what the HN crowd thinks! The premise: even in a world where photo-sharing is everywhere (and free), people are still willing to pay real money for a professional-quality photograph of them doing something really cool (e.g. running a marathon). For evidence of this see: http://www.marathonfoto.com/ (official photographer of the London Marathon). A single image download costs £19.95. But why should "official" event photographers have the monopoly on this? Events like marathons take place on public streets, anybody can take photos. So I imagine a site where professional or amateur photographers can upload photos they have taken and offer them for sale. The upload process would automatically add a watermark to the preview images and display them at low resolution. Buyers can then search and browse photos and download the high-res, watermark-free version. They would pay a fixed fee per photo, which would be split between the photographer and the site. This idea naturally lends itself to big sports events like marathons because everyone is wearing a number, so tagging and searching photos is a lot easier (hopefully it would be possible to implement OCR to do this automatically). However there's really no reason why it couldn't be extended to other use cases (outdoor sports, festivals). Sure the tagging process would be a little more difficult, but photos could be tagged based on colour of clothes, gender, location, time of day etc to make searching possible. To extend the idea further, you could even have a mobile app that people can install, which tracks their location and compares where they've been with the geotags and date info of images available for sale.