What if your content is stolen?

Yesterday I went through a nightmare which I guess every writer goes through atleast once in his/her career - stolen content! I was aghast finding my article published in one of the websites without my knowledge. The individual who published it as her own was in fact one of the buyers in a freelance job website. In her job posting, she asked for a sample article to test my "skills" which I prompty provided. However, though my article was considered "good" by her, she found me too expensive. Nevertheless, that didnt stop her in going ahead and publishing it in one of the articles-for-money websites as her own without my knowledge and also earning from it!!
Not surprisingly, I felt angry, humiliated and violated. I remember getting goose-bumps seeing my article among the list of articles, word by word and finally ending with author's name which didnt spell like mine. It is another thing to have agreed to be a ghostwriter (and seeing your work published) and quite another when your work is stolen. I resolved that the individual has to pay for this.
I immediately plunged in to action and reported abuse to both the freelance job website and the content website where she posted it. I made sure that the content is removed and she is banned from both the websites. Such unethical practices cannot be tolerated anywhere. I had almost prepared myself to take it one step further by posting it on as many sites as possible about the fraud if the websites dont take action. Fortunately, they did and both the content and the individual was removed from the websites. I dont know if the individual in question had any effect but it did help me in getting my content back (and perhaps saving many other innocent writers being duped)!
This, however, has shaken my faith in all buyers when they would ask me for samples. How do we know that our content is not being published somewhere else without our knowledge? Just imagine - an average freelance jobsite has thousands of service providers and most of the projects have atleast 30 bids. If the buyer asks sample articles from each of them, and he likes atleast 15, he can easily go ahead and publish them somewhere else and still not choose anyone! So, he gets 15 articles free-of-charge! I understand not all of them are like that, but are we confident our content is safe? Internet makes it so easy to copy-paste other's work as their own...does copyright really work?
Next time when someone asks for a sample article, how sure you are that it wont be abused? I am not sure anymore...