It hurts the economy more because now people surf for stuff at work, instead of driving from garage sale to garage sale on the weekend.
I also am frustrated with the amount of spam. Try to find a car that's not sold by a dealer, for example. They pose as private sellers on there all the time - just annoying.
That said, my family loves CL. We deal in furniture, kids items, cars, car parts, you name it. It is good to deal in CL in L.A., where you can take stuff off people's hands that are more spendy than you (new furniture every 3 years) and sell your old stuff to people that are even more frugal than you (buying 15 year old furniture).
George