Posting on the internet prevents sales?

The main Share Your Work area here at AW is password-protected, and any work which appears there won't show up in searches and doesn't count as having been previously published.

Other sites don't always take those precautions. Be careful where you put your work.

You can't post here for feedback until you've got fifty posts, but if you spend your time getting to fifty by critiquing other pieces, you're going to learn a huge amount. It's a really good thing to do if you want to improve as a writer.

If you're worried about posting a complete short story, you could always use the beta readers forum to look for a beta reader (or several).

I've posted excerpts of a novel in the SYW forum and feel fine about that, but for some reason posting a complete story anywhere just makes me leery. Plenty of other people do that too, though.

Only two things stop you from being able to sell a story. 1. You sell or give away rights to the story. 2. No editor wants it.

Legal definitins of being published are meaningless. As long as you haven't sold or given away rights, which means as long as no one else owns them, if an editor wants to buy your story, there is nothing at all to stop him from doing this.

The trouble with posting a story online is that editors generally do not want stories their readers may have already seen.

Critique forums, password protected or not, and it's hard to say a site is protected in any way if anyone who comes along can go in and read the story, really don't count, anyway.

Odds are, the version of the story you post is not the version you may eventually sell.

But it's always up to the editor. He can accept any story you own rights to, and he can reject any story, for any reason, including that he doesn't like fried eggs, or hates critique forums.