I do not often sell my copyrights, because this usually means someone wants to make money off my work, without giving me any.  My current policy for copyight is $50 for 4000 words (if you want 5000 words, $55 and it goes up $5 for each additional 1000 words) for a license to use and sell the work (thought copyright remains mine exclusively).  This means that we each may sell/publish the work, but neither has to share any profits or costs with the other (I have a license agreement I can send you to look over).  As part of this agreement I always MUST be clearly credited, as the single and only author of all stories I write, along with the copyright notice.  Omission of credit or copyright notice will constitute a breach and cancellation of our agreement, and resenting of any and all licensing authorizations.