I have posted before about my annoyance at people who ask me to create free samples for them, when I have many samples available on the Internet. It's a rude and unreasonable thing to ask. On Elance (and I think oDesk but I haven't checked), it's against the TOS to require or offer free work, even a mock-up, to be considered in hiring decisions. I recently reported someone for requiring free work (he had a book that needed editing and it seemed he was having every candidate edit a separate chapter...hmmmm) and his job listing was removed from the site.
Recently, someone on AbsoluteWrite posted this link, which gives another excellent reason to not give free samples to anyone. A legitimate publisher, whether in print or on the web, should be satisfied with your existing, already published samples.