No need to reinvent the wheel. This chart, by Spark Capital's Andrew Parker, shows the huge number of fast-growing, successful e-commerce startups that have emerged from mimicking the various functions of Craigslist:
As Alyson Shontell at Business Insider notes, a number of these firms are worth billions. Airbnb is estimated to be worth $10 billion. Indeed was acquired for around $1 billion. Etsy has a $1 billion valuation.
What those firms have done is provide interfaces, apps, and matching and dispute resolution services that differentiate it from Craigslist and help it build a customer base. For example, Airbnb has built its business on attractive photos of the rentals available on the site, and on its dispute resolution and customer service. That's the challenging part of building a successful startup.
But for potential entrepreneurs unsure of what kind of service they want to build, Craigslist seems like a great place to start looking.