It's tough to provide a good incentive. I think your idea of initially targeting zip codes not covered by craigslist is good, but then your problem is marketing to thousands of local, semi-rural areas. Very tough...

Perhaps a way to break into the market would be to find an esoteric genre of items to trade with a very devoted user group - these people generate buzz for free. Then, when you have a group of users, slowly expand into more mainstream categories that have some relation to the original base. For example, you could start with car stereos and speakers, then slowly expand into other car parts, then into everything. I realize this may require shipping and isn't CL-like, so it may not be what you are looking for.

I think the most important thing for me that Craigslist is missing is user profiles of some sort. When I am going to drive 30 miles to some guy's house to pick up a widget, I would feel much more comfortable if other users said he was on the level and dependable.

Feature-wise, I'm unclear on what your site offers that Craigslist doesn't, other than a more web 2.0 interface. If anything, I think a simpler, quicker interface would be better for something like this.