The best answer is this: some editors, like Marion Zimmer Bradley, will not publish reprints of previously published work. Some editors consider online publishing to be publishing, period - and hence will not accept "reprints" of material which has appeared on the Web. Because I want very much to have my work published offline, I choose to provide only exerpts to what I consider my most saleable pieces.
And what about "idea piracy?"
What about it? The fact is, once you have written something - whether or not it is complete - you own it; it's yours. It is copyrighted. Many authors will place additional copyrights on their work, just to be on the safe side. Some of my work holds this additional copyright. Some does not. If someone is planning on plagirizing my online work... well... they're gambling, and it's not advisable to gamble when you don't know the odds. NEVER REPRINT OR REPUBLISH ANY WORK YOU FIND ONLINE, REGARDLESS OF WHO WROTE IT, WITHOUT THE EXPRESS WRITTEN PERMISSION OF THE AUTHOR. Most authors will be flattered, and give permission immediately... so it never hurts to ask.
End of lecture.