While it's true that prospective clients have the right to request samples of your work so they can see what kind of quality they're going to be paying for, there are some unscrupulous marketers lurking in the waters actively aiming at ripping off new writers.
I'm going to bring back Mr. Writer to illustrate how the scam works.
The scam is simple:
Mr. Writer sees a fabulous opportunity listed on Craigslist or on Guru or some other freelance job site. The opportunity always promises that there will be lots of work available for the right writer and always promises decent (if a little low) pay.
The person who listed the opportunity will explain what keywords he/she needs to focus on and then asks any writers wishing to apply to send in 15 sample-articles on those keywords displaying their best work. Of course these sample-articles have to be in great English, approximately 450-500 words and optimized for the keywords specified.
Mr. Writer promptly submits 15 brand new, unique articles and double checks that he's read the guidelines properly. He spends a lot of time getting his application worded correctly so that the client can see he's the best writer for the job.
A week passes...
There's no word about Mr. Writer's application from the person who posted the job. Mr. Writer thinks the poor guy must have been swamped with applications and he's just reading through them all. So he waits a while longer.
Another week passes...
There's still no word from the person who posted the job listing and Mr. Writer is getting worried now that maybe he wasn't successful in getting the position. He opens a new email, intending to create a carefully worded query as to the status of his application when he receives a new email in his inbox.
"Dear Mr. Writer,
We regret to inform you that your articles have been rejected for various reasons. We thank you for your submission, but must unfortunately decline your application at this time.
Mr. I.M. Scam-Master
p.s. if you want to send a new set of 15 sample-articles on these new keywords that I've included below, you may re-submit your application and try again."
:::shaking head in disgust:::
Are you seeing what I'm seeing?
The job listing never existed. Mr. I.M. Scam-Master got 15 unique articles based on the keywords he wanted and then didn't pay for them. Not only has this guy just gotten away with theft, but the job "offer" was worded in such a way that you can't even go after him to get fair compensation for your work.
This is because you voluntarily handed him 15 articles.
Do a search on any plagiarism website for portions of your own text. I'll bet you find several variations of it after Mr. I.M. Scam-Master has blasted your original article through one of those article spinning software things 20 times.
So... the moral of this story is:
Learn to spot a scam at 15 paces. Then run fast in the other direction.
When you stop running from the scam, pause long enough to tell as many writers as you can possibly find to avoid "job-listing" scams like this.