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You don't usually want to sell the copyright, just a use license, which is in effect a copyright rental agreement.
Copyright is actually a bundle of rights that you can sell (rent out) on a piecemeal basis. In other words, you can sell the right to use the same image several times and/or to multiple users concurrently, which is what stock and commercial photography is all about. Few retail photographers "I am a portrait/wedding photographer" seem to really understand the value of their copyright, which here in the US they own exclusively for the rest of their life, plus 70 years.
A use license can allow the buyer exclusive use, which cost more than non-exclusive use. The more media types the buyer wants to use an image in, the more the use license costs, as does the longer the time-frame they want to use the image.
The size(s) they want to use an image at also affects the use license cost.
You can visit
www.asmp.org the web site for the American Society of Media Photographers. On the left side of their home page click on "Business Resources", read all their tutorials and look at their 'Paperwork Share".
You'll also notice they recommend using the commercial photography pricing software fotoQuote Pro 6.