At the time I was a poor grad student and was shopping for a portable AC unit to survive the summer in my sweltering apartment. There were about 5 craigslist sites within driving distance, and I was checking all of them, since saving 20 bucks was easily worth an extra hour of driving at that point. :) Figured a simple thing to learn some basics would be a page that would just spit out the links to multiple craigslist results pages matching a given search.
I literally went to the library and got a book called something like, "Basic web development using PHP and MySQL". The initial site was... very basic. Before I made it I'd looked around for existing solutions, and so when it was done I replied to the few forum threads I'd found, where people were looking for the same thing. It basically grew organically from there. (And over time I made it somewhat less basic. Although the design is looking pretty dated at this point; we're currently working hard on a long-overdue responsive redesign.)
A couple years later I made (an early version of) http://www.autotempest.com, which is like kayak.com for used cars. Then around 2009 I quit my day job and hired my first employee. Have been doing this ever since.