Some websites are offering paid service to post your add on craigslist in large numbers.
they must be using a software that would do that automatically, do you know how are they getting around the system?

Most of those companies that offer multiple postings are just scammers just like the people that post on Craigslist, they just found a different angle. Most likely they will take your money and run. If you want more info on the huge amount of scams on Craigslist, visit http://howtobeatcraigslist.uni.cc