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Kagi. They provide a service that allows you to sell a downloadable product.
You cannot embed a 1-time request for a serial number inside a movie file. Unless you essentially duplicated a DRM scheme like Apple uses with iTunes, it is virtually impossible to protect a video file unless I'm missing something.
You could also make it harder for someone to pirate your video, by embedding it into a web page with Flash or something else that is less accessible outside of the browser (although it's not hard to convert FLVs to another format).