If I post short stories or novel excerpts on my blog, does that make them ineligible for writing contests? How do agents and publishers feel about novel excerpts that are shared in this way? Should I avoid this?Writing contest rules are NOT standardized so there is no way to know in advance if posting work on your blog will make it ineligible for contest consideration.
Faced with this, the savvy writer weighs risk versus reward. Is the reward of posting worth the risk of losing eligibility? Only you will know because only you will know your goal for posting work on your website. Is it to showcase your work? Is it to garner feedback? Is it just to make your website look populated? Is it to build readership or platform? How much do you value the contest prize? Is it money? Recognition? Consideration of your work by an agent or editor?
Weigh all these factors before you post.
And don't worry about posting your work on your website in terms of publication. All too often I hear writers fret that posting their work, or even winning contests, will make their work "previously published."
It doesn't. And publishers don't care even if it did. They're interested in making money and if they think they can with your work, they'll be dangling contracts.
Where you see "previously published" as a problem is when an author has published something that didn't sell well, and thus gives them a track record with booksellers. In other words, there was a book, and an ISBN. Posting stuff on your website isn't either of those.
As to what agents think about posting your work: there is no one answer. I think it's a terrible idea, but it's not any kind of deal breaker.
I think it's a terrible idea because I don't see the value of it.
Feedback? You don't want feedback from unvetted strangers slinking about the internet. The value of feedback is in direct proportion to the person offering it.
Filling up page space? Pictures of your cat or dog, or your armadillo are MUCH better choices. Don't have any pets? Pictures of almost anything up to and including great signs, are better.
The ONLY reason to post work on your website is to build a following. If you're writing a book about writing tips for example, one effective strategy is to post a tip and then ask your readers if it works for them. Building the conversation (and readership) is a big plus for non-fiction.
If you're writing a novel, it's a whole lot more problematic to ask for feedback. And honestly, feedback from just anyone isn't all that useful.
And a lot of people on the internet behave like they were in the movie Steel Magnolias but with a whole lot less wit and talent.
Bottom line: posting your work on your website won't affect querying agents, but it could mess up your contest eligibility. Only you will know for sure if that's worth it.