While it may be free, there is no one monitoring the barter ads, so you must be aware of potential Craigslist scams, and realize that you are always at risk when it comes to meetups and exchanges. For example, about a year ago, I arranged to trade massage gift certificates for housecleaning. Since the individual was coming to my home, I was more nervous than usual. However, she offered good previous references, and we  arranged for a time to meet when my husband would be home, in case an odd, unexpected, or even dangerous situation arose.
On paper, this sounds a bit like delayed barter, but it bears some significant differences. For one thing, it’s much more efficient than Smith’s idea of a barter system, since it doesn’t depend on each person simultaneously having what the other wants. It’s also not tit for tat: No one ever assigns a specific value to the meat or cake or house-building labor, meaning debts can’t be transferred.