Craiglist, Gumtree etc. have large audiences so it's natural for people to advertise things for sale on them, even if their interfaces are not optimal for searc

Craiglist, Gumtree etc. have large audiences so it's natural for people to advertise things for sale on them, even if their interfaces are not optimal for searchers. If you build a competitor you have to build a usable audience of consumers and a decent inventory of products (and a good product). It's not as trivial as it seems at first glance.