Anything that’s posted in its entirety where it can be viewed by the general public is considered published, and the only way you could then sell it to an editor would be as a reprint. That goes for posting to FA, to a blog, to your own website, whatever. The only exceptions are generally for critique groups that are password-protected or where you have to register to see the work. (This is why sites like Critique Circle make you register before you can see any of the current stories up for crit.)
I’ve occasionally skirted this, when I really needed feedback, by posting something to FA as mature, so that only FA members could view it and it wasn’t visible to the general public. Not an ideal solution, especially from FA’s perspective, and that probably only works if you’re trying to publish the piece outside the fandom. (I’ve noticed some fandom calls for submissions now mentioning that things can’t have appeared on FA/SoFurry/etc., so I guess the ‘mature’ workaround wouldn’t work in that situation.)
As far as excerpts, it would depend on how long an excerpt we’re talking about and how much it represents percentage-wise of the whole work. I mean, if something is 40K words and you post the first 1 or 2K as a teaser, that’s not a big deal (and like Sean says above, that’s good marketing once something’s accepted). But if the story’s 5K and you post 2K of it, that’s probably putting up too much. There aren’t any hard-and-fast percentage rules that I’m aware of, but generally you do just want to give a taste, so maybe 10%-20% of the finished word count, max.