What (niche) markets are not serviced by sophisticated online marketplaces?

It seems like there are many niche markets that do not yet have a sophisticated online marketplace developed for them.

For example, it was only recently that Flippa.com emerged (from SitePoint marketplace) as a marketplace catered specifically to the needs of website buyers and sellers.

My question for HNers is this: What markets are in the greatest need of a new, sophisticated marketplace?

Some examples I have considered:

- coin and stamp collecting

- artisanal foods (e.g. cheeses)

- baby products

Thanks!