I think Craigslist/Gumtree/Kijiji are already done for. Their big value came from cross-selling across products and being the one place for everything. This was hugely valuable in the time of Yahoo where content discovery was difficult.
Now with the ease of content discovery the market is fragmenting into specialist sites, look at the profitable areas of Cragislist: housing, cars, dating. They all have specialist sites which are doing better than Craigslist, by taking the most profitable chunks of the market niche competitors are able to spend more per user in marketing and thus beat Craigslist.
In ten years time Craigslist is just going to be left with the least profitable classified ads. You have to ask yourself is that what you want to be competing for ?
Your best bet would probably to concentrate on a particular niche that you want to steal away from Craigslist (local sales, etc.) and figure out the problems people face in that niche and use that to break into the market, rather than trying to compete with Craigslist across the board in what is already a dying model.