Ask HN: Feedback on an idea - Like Craigslist, but open?

Do you think there's a market for a Craigslist-like website, but with an open API which developers can readily use?

Has it been done?

I've been mulling over this idea in the last 48 hours. I think the potential for creative reuse of data is huge (e.g. PadMapper, CarSabi).

It will be consumer-friendly, but the actual target market is developers.