That's sort of an interesting thought. Being the buyer for early sellers as a "user acquisition cost." Clearly this can't be done on a large scale, but I don't think anyone would plan on doing a national craigslist rollout. I wonder what kind of budget you'd have to have to do this for 1 small city for a few months.
Keep in mind you don't have to keep people's stuff, you can always re-sell it on ebay or craigslist :-) (and re-list it on your mythical new site. now you have inventory for the buyers and you are the buyer for the sellers)
That's really an interesting idea, you become the market maker - a monopoly and a monopsony in one, much like amazon is for electronic books right now.
Keep in mind you don't have to keep people's stuff, you can always re-sell it on ebay or craigslist :-) (and re-list it on your mythical new site. now you have inventory for the buyers and you are the buyer for the sellers)