To make it easier to have people participate in your study, you may be tempted to rely on volunteers. But you should not rely on any special types of people, and volunteers are one of those special types. Many studies have shown that people who volunteer differ in important ways from those who do not.
By the same token, if you are interested in testing a particular product, you should not base your decisions only on bad samples just because they have failed. You do not know enough yet about the causes of the failure or the conditions under which it occurred. Conversely, you do not test only good samples because they have no failures. In both cases the results will be erroneous.