The shopping around is one element of the decision, but a key one is the uniqueness of the clip. I think we all agree that iStock has the lowest RPD for video (not sure how that varies with usage and video size, but lets assume they are the lowest). Buyers presumably know that and so if they have an account with iStock, they would start their search there. They will be presented with multiple options for their search and they would normally choose one. Only if none of the options meet their specific needs would they search elsewhere on the next stock site.
So if your video is not on iStock and is not particularly unique then that buyers wouldn't find it and buy it.
Steve