I tried to trade skills using Craigslist and it seems to be a complete failure.
The first reason is the inability to match perception of values. Either people think that your service is more valuable than what they have to offer, or it's the other way around. Instead, people might think that your services worth much less than it is. This leaves a very small amount of people that would perceive your service as you perceive it. You can imagine how hard it would be for two people to find each other and make a trade this way.
Obviously, this is the exact reason we have currency today. It is required to be able to barter and trade good/services. From a business standpoint, money creates an opportunity for easier loss/gain. A salesperson is able to hold gross because of the existence of money. It also allows for loss. Calculated investment risks can be made using money as well. In the end, money is a requirement in a world of capitalism and globalization.
My experience only resulted in contacts with people trying to get it all for nothing. What's worse, was that it was obvious that this was their intent. I'm not sure if I would have ever found an honest trader. Of course, if I did find one, there is a super small chance that I would want what they have to offer. I can imagine that long ago, food was a very strong product to produce, own and sell. Everyone needs it, allowing the seller to approach those that had the service and hold a high chance to close a deal. I would guess he would hold the upper hand at times when the buyer is starving or out of food. Also, many people would approach the seller on their own because of the need for food. The seller would be able to entertain many offers.
In the end, I don't belive trying to use craiglist.com will work when trading, but I do believe a web system could be put together to bridge this gap.