I live in the USA
In the USA there are a number of situations where you would need a model release. The most common involve "rights of privacy/publicity" issues. The laws vary by state, but the general rule is that you need the model's permission to use their likeness to promote goods and/or services (and in some states use their likeness on a product). Note that the issue here is not whether or not
you are making money from the image, but the context of how the image is used. As a general rule if the person is not identifiable or recognizable, then a release is not needed
Imagine that you took a photo of a man sitting on a bench in a public park. You don't need a release to sell the photo to a local newspaper, but the newspaper may need the release if they want to use the photo in certain ways.
A release probably is
not needed in order for them to use the image along with an article about the park administration planning to remove the benches. However they
would need a release in order to use the image in an advertisement selling park benches.
If the person is not recognizable/identifiable, then you probably don't need the release even to use the image in an advertisement. For instance, if you only see the back of the person in silhouette. However even this could require a release if the person's silhouette was distinctive and/or recognizable.
Some people think that a release is not needed for "editorial use", however this is not always the case. For instance a release would be needed to run the photo next to an article about child predators in our parks, as this would paint the model in a "false light" (note: you wouldn't need the release if the model actually was a child predator).
When it comes to selling a fine art print, you generally don't need a release. In some states you would need a release if you were selling lots of prints as then it be a "product". Note that selling a few fine art prints at a $10K each typically doesn't need a release as the image is
not being used to promote goods and/or services.
You need to be careful. While you can sell the fine art print without a release, you may need a release to use that image on an advertisement for the gallery show.
If you are a professional photographer you need to be careful about using the image on your website. If you are using the image to promote your photography business, or the use implies that the model endorses your business, then you may very well need a release. Thus a soccer mom may not need a release to post photos on facebook, but a professional photographer's posting may be promoting his business (which does require a release).
Note that when I say "release", I actually mean permission from the model. Some states require the permission to be in writing, some allow oral permission, and some allow permission to be implied.
Disclaimer: I am not an attorney and this is not legal advice. This is merely an overview of
some of the legal issues surrounding model releases in the USA. The specifics of a situation can lead to a different outcome than the general case. Speak to an attorney for reliable legal advice.