I have sold articles to hundreds of clients over the years, and now I am creating a professional website for myself. I'd like to post some samples of my writing work, but I'm concerned that some of the companies I've sold this content to won't want me to claim it as my own. I've never signed anything that indicated I no longer retain the copyright to my own content. Is there anything stopping me from publishing examples of my work that I've written for other websites?
One example is reputation management work. Obviously, my clients won't want to be identified as clients in this industry. But if I simply changed their names but kept the rest of the content the same, it's possible they could still be identified (eg. if someone copied and pasted a chunk of my text into Google, their web content would come up as the closest result.)
Another example is content published for SEO purposes. If I publish examples of this content on my website after having sold it to a company for SEO purposes, I am reducing its efficacy, because it will appear as copied content.
These are all good questions.
It seems to me that the most important factor for you to consider is the attitude that your clients will take. Never mind who owns the copyright. If a client believes (rightly or wrongly) that they are the copyright owners, they will feel justifiably aggrieved if you go ahead with the posting - especially if you do so without their permission.
Hiding the clients' names doesn't really change that. If the client recognises the content as theirs, the damage will be done.
The obvious solution would be to contact the clients, explain the situation, and ask their permission to post the articles. Point out that you will give the client an acknowledgement, and include the client's copyright notice if there is a reasonable possibility that they are entitled to that. Be sure to tell them that you are doing this only to show a portfolio of your work, not to compete with the client in any way.
I doubt very much that any of your clients will object to that.
As far as duplicate content is concerned, that's an overblown issue. Having more than one copy of a given piece of text is much less of a problem than many people suppose. However, if the client is likely to be at all concerned about that, you can offer to add a noindex attribute to your copies, to ensure that they won't show up in the search engines. That will satisfy the client's concerns, and it won't do you any harm either. After all, your aim is for your site to be seen by your potential clients - people who will be searching for terms like "freelance writer" or "copywriting in <name of city>" - not those searching specifically for the articles in your portfolio.
I agree with Mikl. You should just contact the people you have written for and explain that you are going to use what you have written for them in your portfolio.
I would contact the clients first. And, if allowed, publish the content as .pdf (or anything that won't make it duplicate, those people didn't pay to have their content displayed on other sites).If this doesn't work, create some content for 'virtual' clients/businesses and you have something to showcase in your portfolio.
My thoughts are you already knew the answer to your question before you asked it.
Why not use their site as a reference instead and link it from your portfolio? You don't have to say what you did for them or point to it.