Regarding scope of use/copyright, the trick is to be clear what you are selling. Agree with the purchaser what rights they are buying - are they buying the right to publish your photo in traditional paper-based media (books, magazines, posters, adverts - some or all of these?), or on the web?
Is that right to publish exclusive, i.e. you cannot sell the same photo to another purchaser for the same type(s) of use, or non-exclusive? Are the rights territorial, i.e. can the purchaser print your photo only in US- or European-published media, or worldwide? Are the rights permanent, or time-limited?
Make sure that the purchaser knows EXACTLY what rights they are buying by setting out those rights in a written, pre-sales agreement. Broadly, the more rights they buy, the higher the cost.
I am based in the UK, and I was always advised not to sell all rights (i.e. world rights, all media) to any photo for less than GBP500=USD875.
When submitting written work with accompanying illustrations to magazines, I have always accompanied the submission with a statement to the effect that "The enclosed material is offered for a single publication in [name of publication] at your usual rate of payment. Copyright in all enclosed material, both written and photographic, remains with the author unless expressly agreed to the contrary in writing prior to publication. Publication of the enclosed material indicates your full agreement with the above conditions."
Hope this helps,