As a rule, it is best not to give samples unless you have written them for yourself and secured all necessary copyrights. The vast majority of people who ask for samples go on to ask for more specific samples. Either this or they just don’t convert.
When buyers ask me for samples I say something like:
With all due respect, I am a freelance ghostwriter. I could send you links to articles or cut and paste articles from anywhere on the Internet and say I wrote them. I would be happy to write a test article for you for (insert $$ amount here). However, I am afraid that I do not give away free samples.
You would be in violation of a past clients copyright if you use work which you have delivered to them as a sample without their express permission. Also, many people asking for samples will be resellers who intend to use them as samples in their own portfolio. - In short, it’s up to you if and how you make samples available. Just be aware of the risks and never give out samples to people with atrocious spelling and grammar.