Craigslist has one of the worst UX. How come has it not been improved in years? Why doesn't Craigslist get any competition? It should be trivial to implement something that's 10 times better...
Listings can be a mixed bag, but that's entirely user data so it's hard to fault Craigslist too much. IMHO it's better than having some 5 page form you have to fill out every time you list something which people will just screw up anyway.