the site has to have one of two compoments - that are really one
People pay to list on eBay, even though there are a zillion free
places to post because eBay has an enormous community of buyers and
the likelyhood of selling your item is good. With the exception of
craigslist, there are few places that offer this.
So, if your sites new or lacks an established traffic flow of buyers,
then it's nearly impossible to sell listings. Unless the listing fee
is used to attract buyers.
If I was going to try and start a paid classified website, I'd try and
build a community first. Microsoft tried to launch a free classified
website on the strength of its funnel. I doubt that this service has
the stregth to take on craigslist. However, Facebook launched a
classified site on top of its community. I think this has a good
chance of becoming the dominant classified site.