Competing in the low end means that its a volume based game because each unit of value (each craigslist posting) is low. A new company trying to gain momentum capturing a small user base does't work because in this smaller stage of the company you have no network effects.
If you try to steal those low end customers with a 10x product, you will spend a ton of money to capture users that aren't very valuable.
If you make something that adds 10x more value, with people who want 10x in value, then by design you work yourself out of Craigslist's turf like AirBnB (they started hacky low end but now are moving into higher end listings to drive growth).
in short, I believe there is a valley of death when you try "siege tactics" on Craigslist's network effects.