We just did YC S17, and another company in our batch was Fat Llama, which is a rent/borrow marketplace for tech gear. Think DJ turntables, drones, and high-end cameras.
Haven’t used it yet, but makes sense for people that only need the gear a few times / year (and for those who always buy they hot new things, but rarely use them).
Also our company, Sixty, is a 3-sided marketplace. We build partner programs as a service for SaaS companies by providing them tools (software) and talent (vetted freelancers) for an on-demand marketplace.