It always surprises me to find out that many seasoned writers are not aware of the article checking site, Copyscape. At Copyscape, you can check to see if the article you have just finished is completely free of plagiarism, copying someone elses work. Now, I am not saying that you intentionally copy others work, no honest writer tries to do that. But sometimes, as we write our articles after researching several sources, we may unintentionally copy a phrase or line that can come up as plagiarism later on. By checking your article, you can make sure every article you submit for sale is yours entirely. Most writing sites now use Copyscape to check your articles, so it's better to know first if you have to re-write a line or two than have the site think you copied it. Copyscape isn't free, you must pay .05 cents for every search, but it is well worth the trouble if you are as serious writer.
There is a free alternative to Copyscape, and that is ArticleChecker. You can paste in your text for free and it will check both Google and Yahoo. Whichever site you decide to check your articles with, I think it is important to use one of these sites to make sure all of the work you submit for sale is yours, free of plagiarism. It is good insurance to keep your good name as a writer.