I don't think most website owners actually know how to check for dead links on their websites, and most probably don't care.

You can utilize dead links/dead websites for Search Engine Optimization purposes, either by buying the old website that is now defunct, or by finding out what they previously linked to and creating a new page on your website with similar but better information, and letting the owner know about their dead link and asking them to link to your site instead.

There is an industry in this segment, but maybe you can figure out a better way to capitalize on it.